Instructors matter if they are into "jumps" and similar utter nonsense. You begin to wonder why you'd bother with a class if just about everything you do is going to be completely different from what the instructor is suggesting. You might as well just get on a spin bike in another part of the gym and do your own thing entirely.

But get a good instructor, and yes, you can adjust your resistance and cadence as you like ... and still follow the program that they're teaching.