The Feb. 2008 issue of Consumer Reports rates health clubs, with the worst rating going to Bally's Total Fitness.
Bally's got bad marks in staffing, classes and equipment, plus the worst possible score for locker rooms, cleanliness and crowds. On the flip side, the best club is a chain that's in 15 states called Lifetime Fitness. In the second-best slot are on-site gyms at work and coming in third best were gyms at your local Jewish Community Center/YMCA/YWCA.
Though Consumer Reports doesn't mention them, I also recommend checking out hospital-affiliated fitness centers. They're usually rehab-based or geared toward hospital staff. They're clean, well run and don't force stinking contracts on you. Most sell memberships to the public.
One final word: Signing a contract will not get you to work out. You may have the best of intentions, but most people quit working out within weeks. So don't obligate yourself to a multi-year contract.