Played this with a smaller number of people (6-6) and have feedback based on that. First of all, everyone was really excited about it. It's one of the better starts to a payload map that we've seen, and we've done extensive playtesting on es_cave/pl_cave for Spacemonkey and played all of Youme's hoodoo releases. I won't say it's as good as hoodoo was at this stage, but it does definitely have the potential to be as good.
Now, onto my thoughts which are based on talking with everyone as we played. I'll start with some more generic comments.
1. What an excellent map for spies. There are a ton of hiding spots and some nice overpasses to attack from.
2. Engineers will definitely be useful, but I think you should reduce the "shortcut" path somehow. Right now the attackers gain too much ground too quickly, which I'll explain with the image below.
3. It's pretty well balanced for each of the classes, but I think the first half will be very spammy because it's so narrow. I'd suggest trying to make it wider in the first half of the map and leaving the second half.
4. The balance is a bit off because of #2 and #3 right now. It's hard to tell with 6-6, but I really think with a full server the attackers will have an impossible time getting to the first checkpoint. Once they get there, however, it's pretty much a free ride until they reach the last 1/3 of the map. I think this is a result of the spawn positions, shortcut accessibility, and spawn timers. A number of defensive sentries might balance it more, but you don't want to force the defenders to have more than 2 sentries.
5. The last 1/3 of the map is the most fun. I think it's because the layout is pretty simple, but it still allows the attackers and defenders a variety of paths/locations to choose from. I think the first 1/3 of the map would be more fun if it were wider and simplified a bit.
6. The explosion should either be much larger or shouldn't hurt people standing on the tracks above.
7. The rear spawn for red is a bit small and cramped on the way out. I'd suggest widening the path to prevent spawn camping (let the people move around a bit more on their way out).
8. The edge of the quarry is fun for building hidden teleporter exits, but is somewhat useless otherwise. It has the potential for being one of the most fun areas of the map too. I'd suggest doubling the length of the tracks into the quarry or exposing the tracks more at that point, which would allow sentries down below to hit people on the tracks above.
Here's the picture...
Yellow areas: These are where most of the fighting occurred. Each time we played it, we jumped from that first section to the second with very little fighting in-between. I think this would change a bit once people know the map (and good sentry locations), but it was pretty apparent that something was still off.
Red areas: I don't remember being in these areas at all. I think the first red area is the shortcut for the attackers, but if not then both of these areas got very little action. That may be because we didn't know the map yet, or it may be a result of there being no need for us to go in those areas since the attackers cruised right through with little resistance. I'm sure that's not useful, but I figured I'd mention it in case someone else felt the same.
Pink areas: I said that the last 1/3 of the map is fun because of the simplicity of it. These areas are not fun for the opposite reason. It's obviously hard to tell what your goal is with no texturing, but these areas were still too complex. In the first one, instead of having multiple ramps including a switchback ramp, you might want to just make it an inclined area and put a single stairway inside the building (if the design allows). There's also a gap between that area and where the cart starts that's too small/high for a player to get through, but too low/unprotected to be useful for sentries or attacking from above. I think this area just needs some simplification. The second pink area is similar, with stairs going up, half walls, etc. Those half walls (on the upper level) are positioned in a way that they're useless for defending sentries and only get in the way of defenders. The lower platform is impossible to jump onto from the ground, especially when you're in the middle of fighting. My recommendation would be to greatly increase the width of this area, add small steps onto the lower platform, and simplify the upper level.
Like I said, we didn't have that many people on the server and didn't know the map, so take this feedback with a grain of salt. Hope it helps, though.