I actually did use the BC gym last year. I meant I only went to main campus twice for other stuff. I agree with you that its pretty awful. There are no TVs, and there are limits of 30 minutes for most of the cardio machines. It can get really crowded at peak times too, so there is sometimes a wait for machines. The nice things about it are that it is really cheap for law students. I think I paid like $275 for the whole year or something. You can use the pool, tennis courts, and the other facilities, and it has the indoor track, which none of the other gyms in the area have.
I'm not rejoining the main gym this year though, and will hopefully find another gym, which isnt too expensive. When I was looking for gyms last year, I looked at the oak square YMCA, which is really nice and is $56/month and the Bally's in Brighton, which is really nice and cheap ($30/month). The problem with Bally's was that they make you sign up for a 3 year commitment, and its a little farther from the law school, and since I knew I would be moving around throughout the 3 years I wasn't sure I would always be close. A few of my friends belong to the Beacon Hill Athletic Club and Boston Healtworks, but I think those are more expensive ~$70/month. It also sort of depends on where you plan on living.
There is a little law school gym, but it was under renovation all of last year, so I never got a chance to see what they did with it. It's REALLY small, and I have a feeling its going to be like a treadmill and a basketball court or something, and maybe some freeweights, but I doubt it will be anything to write home about. I could be wrong though. It's definitely worth checking out before signing up anywhere else.