All about the instructor. They choose the routine, they choose the music, they are "mono-logging" most of the class.
You might groove with it and get a great workout session. You might just cringe for an hour.
There's one nearby that I just won't go to ever again. Not only too much Katy Perry, Cyrus, GaGa,, but she constantly sings along with it into her earpiece mike. Not a singer. At. All.
"OK, THIRD POSITION! FIVE, FOUR..."
Wrong kind of suffer-session.