I'm picky about gyms so I've tried a good many on this list 
Reviews as follows:
1- Haosha: Haosha branches are super inconsistent. There is an OK Haosha in Dongdan and also a decent one at Dongzhimen Bridge (there is a pool included at the latter but it's in Gongti, which I find annoying). However, one consistency seems to be broken equipment. At each location I've been to (about 2-3), there have been "equipment malfunctions" at least twice a month wherein half of their treadmills or ellipticals-- as well as their locker room facilities--would be broken for a few days. If all you're looking to do is swing weights around though, Haosha might be good for you.
2- CSI Bally: I've been to the Wudaokou, Wangfujing, and SOHO CSI Bally's. All have been pretty good with decent rates. Their rush hours get more crowded than at other gyms. The Wudaokou locker rooms are terrible and the SOHO location is an improbable three-story building. It is also the only location to have a pool, but it's not worth it, particularly with..ahem, pool etiquette being what it is here.
3- Pacific Century Club: not worth it. Sorry. Also, treadmills are crap.
4- Nirvana: It is a toss up as to whether or not the rates for Nirvana are worth it, especially since they raised their prices a couple of years ago. The Jialong Fandian Nirvana is OK. The Wu branch isn't that great, except that it has a killer Friday evening spinning class. The clientele is obnoxious. Locker rooms have a hilarious "stand up dryer" in case you forget your towel.
5- Powerhouse: the staff has a piss poor attitude and will try to make you pay RMB 4000/year for halfway decent equipment and lockers that smell like death. Find someone who already has a membership and get them to hook you up with their membership consultant for a better deal. Also, if you agree to go during "non peak hours" they'll usually give you a better rate. Don't use the showers. No, seriously, don't use the showers.
6- Aomai Fitness at Sanyuanqiao is decent but located underground, so claustrophobes beware. Has possibly the nicest pool I've been in in Beijing. Locker rooms are decent.
It also should be mentioned that a lot of the "perks" these gyms offer, i.e., hot yoga, are not included in membership fees. You need an extra card for them.
Hope this helps!
