I've been a member at Curves for almost 3 years, although i continue to struggle to get there regularly. This is no fault of Curves, but my own.
I got in on a special deal where my joining fee was greatly reduced and currently pay $31 per month(including tax). As far as i know it's no more than 37 per month(including tax) if you join now and i'm in the Seattle area where we have a pretty high cost of living.
When i go 3x aweek and make sure that i'm keeping my heart rate elevated, not just going thru the motions, i lose inches and lbs and gain strength.
The way the machines work is the harder you push the more resistance you get and the more you work your muscles. Supplementing w/some free weights is helpful i'm sure depending on what muscles you feel need more work. I like that i can go and work hard and get a cardio and muscle workout all in about 30-35 minutes. I actually prefer it to the machines with the weights that you have to continually adjust and i like that it's for women only as i feel less self-conscious in that environment.
I've thought about quitting to save the money and just walk and work out on my own, but i know that i need the structure and support that i find there. Of course each owner manages in their own way so all curves are not created equal in that sense. I'm fortunate to have found a good one where the owner is very dedicated to helping her clients really get the most out of their membership.
I'd recommend really giving it a chance during those 30 days. There approach is backed by research. I think there can be more than one valid way to do a strengthening workout and i think Curves is a good one for a lot of women.