I took the MCAT in Boston last year, and so did a few of my premed friends. I know general info about four sites:
1) The Hilton. I was here, and it was decent. Tons of people, started 45 min late due to setup, but pretty efficient. We had big folding tables shared between two people apiece, but they were particleboard which made erasing a pain. Proctors tried to be efficient (about as much as you can with 200 or so students) and we only ran about 1.5 hours late, which wasn't too bad. They weren't evil about having drinks under the table, which I've heard can depend on the proctor.
2) Tremont. Most of my friends were here, and it was a disaster. They ran four hours late and the Roxy was doing sound checks the whole time. And since the exam is always on a Saturday, it's likely they'll be doing sound checks this year as well. I'd avoid it if you can.
3) MIT. All I know about this is that they had tiny desks in the past. We were told to avoid it if that bothered us.
4) Wellesley. Not in Boston, but completely worth it. I think they only take 40 students, so it's about as fast as possible. You get big spacious tables to yourself, the campus is suburban, beautiful and relaxing, and the building is quiet. The only hassle is getting out there and getting in, which is just luck of the draw. Kaplan rated in #1 in the country last year and you can bet all the Boston students remember that.
Hope it helps.