Well there isn't really such thing as a Recumbent Bike "class" I don't think - but back injuries are definitely BAD. That is what kept me away from exercise for a week.
My physical therapist said that people with back injuries should use the recumbent bike because it not only puts you in a more ergonomic posture, but it strengthens the core muscles, too. I AM NOT willing to risk a recurrent back injury that will put me on the sidelines again.
I've been using the recumbent, and you can set it up for "Interval Training" -similar to the type of fast-rpm-then-slow-rpm that the spinning classes offer. Maybe you can even bring the recumbent into the class auditorium and still participate in the class atmosphere. The one I use overlooks the valley, and the sunset and I just crank up the music and pedal away without fear of injury. It measures heart-rate, too so you can use the same intervals etc. But, I'm not really a "class" person, so I don't miss that part.