I like that no matter how I feel before I go I feel like a new man after my workout. I also love that after a stressful day I don't have to think but just 'do' and let the instructors take over. It can be very zen. If a room full of people with music blasting and an instructor yelling to keep running faster can be zen.
I love how the workouts always push you. There are days when I go to work out thinking I'll just take it easy, but with the coaches enthusiasm and the tenacity of the other people working out, I always push myself to do that extra rep or increase the speed on the treadmill by 1 or 2.