I like it.
The workout is as good as you make it. I usually go around an extra time, so it's a 45 minute workout instead of a half-hour. If you really work the machines, they will work for you.
For a lot of folks, the challenge is just to keep going to the gym. That's where Curves helps, because there is a large social component. Each location is a little different, but Curves tends to have a chatty culture, especially if you go at regular times and see the same people.
The owners also try to make it fun. There's often some sort of trivia contest, random drawing, food drive, product trials, or some-such happening.
Curves has definitely helped tone my arms and legs. Maybe a gym with serious free weights would get better results, but I have joined gyms in the past and not stuck with it.