Played in UGC Iron as an engineer. We played the b8b version, and won 4-0 against a similarly placed team.
First thing I'd like to say is that no shutter doors are nobuilt in Valve maps - you can do exactly the same trick as stopped in the b9 update. Most obnoxious location is actually on Badlands second: there's a huge sightline from the door to choke that you can wrangle. Still, I'm the real comp guys don't know that because they don't play with engineers. B)
I have not played b9, but I think that dropping the capture time from 24 to 16 seconds will have a huge effect on the gameplay. I don't know whether it will be good or bad, just different. I mean, it's going from one of the longest cap times in the game to one of the shortest - about the same length as lakeside. If I were to guess from how we played b8b, that will stop teams from holding passively when owning the point and pushing back in when the other team starts to cap. In other words, teams will hold further forward and that means snipers are more powerful. That probably makes it even more similar to Viaduct, but the width of the map will mean that good work from the flank classes will hold the sniper down better.
On another note, minisentries were insanely powerful on b8b, especially in desperate scenarios. That's because they could be placed on the sides of the map with height advantage, resisting their destruction, and the long cap time meant that you couldn't escape their fire while still maintaining the push. Dropping the cap down to 16 seconds is probably wise from that point of view.
I might update this post later with other thoughts from my team.