I have taken lots of "group exercise" classes over the years. It has been my experience that the pre-choreographed routines (step, spin, kick-box or whatever) are usually "OK" but not great.

A great instructor teaches a better class than the canned routines. But the canned routines are better than the bad instructors!

But even in pre-choreographed classes, some instructors that are better than others. If you're lucky, you have choices and you can pick what works for you. But if you're not, you take what you can get and make the most out of it.