I agree a warmup is definitely necessary which is why I usually get to class 1/2 hour ahead of time (earlier if possible) and start spinning. Basically the way my class is run it's 1 1/2 hours if you include a warmup, the class and proper cool down after. We are all on our own for the warmup and cooldown. If I see a newbie come in, I explain that the class, once the instructor gets there will be off and running so start spinning now. I generally ride another 45 minutes (thanks to my trust iPod) after class at a lower intensity.

I like the concept of the Spinerval Traning and doing crunches etc during the workut. If i think about it, I do crunches after I'm done spinning my sometimes get lazy.