It is expensive I suppose, compared to regular gym memberships. However, the fact that a) it's right by my house, and on the way home from work plus, b) the workout is just 30 mins means that I actually go far more regularly than when I used to go to the gym.
It's just easier all round. I found I went religiously to the gym for the first 2 months, and made full use of all their facilities (pool / sauna /steam room etc) but the entire trek was taking me over 2 hours. This typically made it difficult to go more than 2 nights each week. I ended up going perhaps once a week at most...
With Curves, I'm in and out of there with no hassle, and have plenty of time to get on with other things.