The interval training, different routine every day, awesome coaches and awesome music. Very positive environment. I've been an athlete all my life and it's the only workout where I leave feeling, day after day, that I've given it everything I've got.
I love the variety and the difficulty. Every day is something new, so those no boredom or getting stuck in my fitness achievements. I feel they push me to get better, stronger, faster and have fun doing it.
Everything! I like the encouragement from the trainers, changing up what we're doing the moment it starts to get too hard or boring, the fact that my body is working so hard that I can't even think--inadvertently giving me an hour of zen, that I'm competing against myself and whether you look like me or captain America we're both sweating the same amount trying to get to the orange zone--and the orange lighting that disguises just how red and sweaty my face is :)