The quality of circuit training can vary dramatically.
Also, depending on what you're training for, e.g. weight-loss/hypertrophy/athletic performance, circuit training can mean completely different types of classes.
In general though, the type you described (Curves/Fit 2 Go) is better than nothing, can be fun (which is always good!) and has a social element if you bring your friends, which can all help in exercise adherence. It's a great beginning. However, as you advance, you will need more well thought out classes and better training.
WARNING: I heard that Curves especially uses some god-awful
weight training equipment - Cursor knows more than me about it - try a seach for a thread on curves - I remember he posted in it.