I just joined Curves 3 weeks ago. I view it as a temporary aid. I like the circuit set-up and it gets me in the habit of working out regularly. I think it's good for someone who is out of shape, very overweight, or some elderly. One lady here uses a cane to get from machine to machine, and can barely do the exercises, but it's inspiring to see her motivation!
It is definately helping to tone my out of shape muscles, but it's not super challenging. It's $34 or $35 a month, plus tax, and it's on a month to month contract. It's good for what I'm looking for at the moment, but I expect that by fall, I'll switch to a gym.