Where in LA do you live, and which area do you plan on working out in? Some gyms have only one or two branches but are great.
24 Hour Fitness has TONS of locations in LA, they're always having promotions, and are generally very good. I've been a member for a few years now.
Bally's has outdated equipment and is downright filthy. Stay away from them.
LA Fitness has been expanding a lot lately but I've never been in one and I don't know anyone who goes there.
Crunch gym has a few locations and they're supposed to be the end-all as far as gyms are concerned, at least according to the large amount of celebrities that work out there.