This discussion started in the Furnace Creek thread, but I figured I'd save Nineaxis and Youme and get it moved out here. 
First, why I feel I can comment on this. I play a LOT of engineer. I'm over 200 hours as an engineer and it's my most used class by about 4 hours. More specifically, I play a lot of offensive engineer and mostly play spy on defense. Those choices have allowed me to see sentries from both sides as a good player with both classes. I also play on a regular basis with CEVO-Pro and CEVO-Free players, so I'm not playing against complete scrubs. One of my clanmates is considered one of the better demomen in the game and I've played with many of the other top players.
My initial rebuttal to Mar's posts...
In the other thread Mar tried suggesting that sentries are crutches for poor players. This is completely ignorant. Sentries can be crutches for poor players just like stickies can be crutches for poor demomen, medics can be crutches for poor heavies, and w+m1 can be a crutch for poor pyros. However, sentries can also be a very effective tool for excellent engineers.
In the thread he also claimed that sentries are extremely easy to destroy, but that they grind the game to a halt. If they're so easy to destroy, then why would they grind the game to a halt? Those statements are contradictory.
The role of sentries...
First and foremost, the role of sentries is area denial. Obviously once a sentry is up, smaller classes have a very hard time controlling that area. This is especially important now that scouts are so powerful... the sentry is the last complete counter to the scout class. Heavies move so slow that the Sandman makes them useless. Good scouts can easily avoid soldier's rockets and pyro's flames. So, the sentry is the only thing left in most situations.
Second, the role of sentries is to force early ubers. A good example is putting a sentry near the exits on Dustbowl in order to force the enemy to uber immediately. That gives them far less time to actually attack the control points. This is very similar to CEVO players focusing their fire on the medic. The medics are so good it's rare that you're able to actually kill the medic, but you can often force them to uber before they really want to.
Third, the role of sentries is to provide a distraction. This is #1 for me as an offensive engineer. I drop a sentry and separate from it immediately when facing a single enemy. That enemy is forced to come after me or go after my sentry. If he comes after me, there's a good chance my sentry gets built and starts killing him. If he goes after my sentry, there's 2 free hits with my shotgun. If there's metal around while I'm fighting I'd pick it up and repeat if he destroyed my sentry. Most players have a hard time dealing with this.
Fourth, the role of a sentry is to get a few kills. In most cases I'll only leave a sentry up a few minutes max, unless it's in a highly valuable position. It's far better to move a sentry around and get multiple surprise kills than to have to hide behind it with my wrench the whole round. If you kill 2 players per sentry location, then you're being an asset to your team. More than that is just gravy.
Fifth, sentries direct game action. If you have a good defensive sentry position in a map, you'll force heavy/soldier/demo/medic/pyro action to the edges around that area. You'll also force scout and sniper action away from those areas. It's a good way to get people to play areas of your map more by adding sentry locations on the outskirts of those areas.
What this means...
Obviously this means that there are uses for sentries outside the classic static defense role. Making a blanket statement that maps shouldn't get good sentry locations is stupid. That makes scouts and pyros far more powerful, makes it nearly impossible for teams to recover from an enemy uber, and reduces the effectiveness of friendly spies (which many times backstab a stationary demo/soldier trying to take out a sentry). Sentries add depth to the game. Without them it turns into a massive team deathmatch.
Why is this in the mapping section?
Because map design determines whether sentries are useful or not on maps. They also determine which of the roles they fill on a map. If you want players to focus their attention on a certain area, add a protected sentry location. If you want to give defenders a chance to recover from ubers, add a sentry location. If you feel that sentries can be too powerful in certain areas (Dustbowl 2-2), move the good sentry locations to the outskirts of that area. That allows engineers to focus on popping ubers while giving the attackers the ability to push through and then attack the main area without facing massive sentry farms.
Surprise sentry locations in tunnels are great for this, as are alcoves that are easy to build in but hard to defend. Open areas with metal in select locations allow engineers to use the distraction method with their sentry. Finally, making good sentry locations that cover a limited number of choke points allows engineers to lock down those routes. As long as you have enough routes in, engineers won't be able to lock them completely down. A good example is Gravel Pit C, where you can cover 1-2 exits with sentries, but covering them all would require over half your team to be engineers.
A good sentry location will have a clear line of sight across an area, completely to the other side (don't make the areas too wide). It'll have metal really close and will sometimes provide an escape path for the engineer in case his sentry is getting destroyed. To make them far more effective, they should be placed below dropoffs, on the back side of common routes (so you hit people as they run past), and underneath overhangs (Gold Rush 2-2). Those things should be decided based on the balance of that area.
Hopefully this will help some of the mappers make decisions about sentries and sentry locations.
First, why I feel I can comment on this. I play a LOT of engineer. I'm over 200 hours as an engineer and it's my most used class by about 4 hours. More specifically, I play a lot of offensive engineer and mostly play spy on defense. Those choices have allowed me to see sentries from both sides as a good player with both classes. I also play on a regular basis with CEVO-Pro and CEVO-Free players, so I'm not playing against complete scrubs. One of my clanmates is considered one of the better demomen in the game and I've played with many of the other top players.
My initial rebuttal to Mar's posts...
In the other thread Mar tried suggesting that sentries are crutches for poor players. This is completely ignorant. Sentries can be crutches for poor players just like stickies can be crutches for poor demomen, medics can be crutches for poor heavies, and w+m1 can be a crutch for poor pyros. However, sentries can also be a very effective tool for excellent engineers.
In the thread he also claimed that sentries are extremely easy to destroy, but that they grind the game to a halt. If they're so easy to destroy, then why would they grind the game to a halt? Those statements are contradictory.
The role of sentries...
First and foremost, the role of sentries is area denial. Obviously once a sentry is up, smaller classes have a very hard time controlling that area. This is especially important now that scouts are so powerful... the sentry is the last complete counter to the scout class. Heavies move so slow that the Sandman makes them useless. Good scouts can easily avoid soldier's rockets and pyro's flames. So, the sentry is the only thing left in most situations.
Second, the role of sentries is to force early ubers. A good example is putting a sentry near the exits on Dustbowl in order to force the enemy to uber immediately. That gives them far less time to actually attack the control points. This is very similar to CEVO players focusing their fire on the medic. The medics are so good it's rare that you're able to actually kill the medic, but you can often force them to uber before they really want to.
Third, the role of sentries is to provide a distraction. This is #1 for me as an offensive engineer. I drop a sentry and separate from it immediately when facing a single enemy. That enemy is forced to come after me or go after my sentry. If he comes after me, there's a good chance my sentry gets built and starts killing him. If he goes after my sentry, there's 2 free hits with my shotgun. If there's metal around while I'm fighting I'd pick it up and repeat if he destroyed my sentry. Most players have a hard time dealing with this.
Fourth, the role of a sentry is to get a few kills. In most cases I'll only leave a sentry up a few minutes max, unless it's in a highly valuable position. It's far better to move a sentry around and get multiple surprise kills than to have to hide behind it with my wrench the whole round. If you kill 2 players per sentry location, then you're being an asset to your team. More than that is just gravy.
Fifth, sentries direct game action. If you have a good defensive sentry position in a map, you'll force heavy/soldier/demo/medic/pyro action to the edges around that area. You'll also force scout and sniper action away from those areas. It's a good way to get people to play areas of your map more by adding sentry locations on the outskirts of those areas.
What this means...
Obviously this means that there are uses for sentries outside the classic static defense role. Making a blanket statement that maps shouldn't get good sentry locations is stupid. That makes scouts and pyros far more powerful, makes it nearly impossible for teams to recover from an enemy uber, and reduces the effectiveness of friendly spies (which many times backstab a stationary demo/soldier trying to take out a sentry). Sentries add depth to the game. Without them it turns into a massive team deathmatch.
Why is this in the mapping section?
Because map design determines whether sentries are useful or not on maps. They also determine which of the roles they fill on a map. If you want players to focus their attention on a certain area, add a protected sentry location. If you want to give defenders a chance to recover from ubers, add a sentry location. If you feel that sentries can be too powerful in certain areas (Dustbowl 2-2), move the good sentry locations to the outskirts of that area. That allows engineers to focus on popping ubers while giving the attackers the ability to push through and then attack the main area without facing massive sentry farms.
Surprise sentry locations in tunnels are great for this, as are alcoves that are easy to build in but hard to defend. Open areas with metal in select locations allow engineers to use the distraction method with their sentry. Finally, making good sentry locations that cover a limited number of choke points allows engineers to lock down those routes. As long as you have enough routes in, engineers won't be able to lock them completely down. A good example is Gravel Pit C, where you can cover 1-2 exits with sentries, but covering them all would require over half your team to be engineers.
A good sentry location will have a clear line of sight across an area, completely to the other side (don't make the areas too wide). It'll have metal really close and will sometimes provide an escape path for the engineer in case his sentry is getting destroyed. To make them far more effective, they should be placed below dropoffs, on the back side of common routes (so you hit people as they run past), and underneath overhangs (Gold Rush 2-2). Those things should be decided based on the balance of that area.
Hopefully this will help some of the mappers make decisions about sentries and sentry locations.