I've just got the qualification to teach circuits. If you enjoy it and you feel it's working you hard, do it. As a teacher the type of thing you can do in circuits is endless, so whether or not it's effective very much depends on what the teacher is doing. If it's aerobic fitness you're after, stick on your HR monitor. A class I went to on Tuesday night passed as a speed session for me - albeit a short one. As I'm training for a marathon, I do sometimes feel it's a cop out, and try to include it as extra to my schedule, not as a replacement. Anyway, I waffle! If you enjoy it, and you've got time to fit it in, you might as well just do it.