The 24 hour fitness at Market and Church is a small gym and it's always pretty crowded. It's like an Active Sport sized gym.
The 24 hour fitness at Van Ness and Post is a full sized gym (but not a Super Sport) with great equipment. It does get busy, and I suspect that it will for Folsom, but people do move through the equipment. It's best to avoid "rush hours" if possible. Towel Service here.
The 24 hour fitness at 100 California Street is where I usually go. It's not as big as Van Ness, but it is a full featured gym. It's also best to avoid "rush hours" if you can, especially during the week to avoid the Financial District crowd. Look for "Aaron" at the desk. He's the hottest attendant! drool! Towel Service here.
The 24 hour fitness at New Montgomery is (from the last time I went) as smaller gym, kind of like an Active Sport.
The 24 Hour Fitness at Portrero is a full sized gym. I haven't worked out there. I've only visited to check it out. It is relatively close to Folsom Street.
The 24 Hour Fitness in the Marina District is an Active Sport gym.
The 24 Hour Fitness at Berkeley is about the same size as 24 Hour fitness 100 California Street.
The 24 Hour Fitness at Dublin/Pleasanton BART is a Super Sport. No waiting, but they will charge you $10 extra if you do not already have Super Sport on your membership.
Make sure you bring your own combo or key lock. Make sure you bring a towel. Some of the gyms have towel service and may toss you one as a courtesy if you're from out of town, but don't count on it.