I think it's better to do a regular group class, like circuits, spin, boxercise, etc, if you want pushed harder than you would push yourself in the gym, but it seems more fun than PT.

I've been doing circuits for a few months and all my fitness figures have shot up. I lost 4 inches off my waistline, blood pressure perfect, BMI spot on, heart rate, everything. Achieved all my targets (and I'm quite a heavy smoker).

I go to the gym other times just to maintain that level.