I used to spin over the winter 3 times per week and still maintained it once per week during the summer months. You will find the first class to very tough so don't go at 100% otherwise you'll puke. However, being a cyclist you'll catch up and keep pace with the rest of the class in a few short weeks if you decide to stick with it.
I definitely enjoyed the workout s while I was in the middle of it I was in fantastic shape and I found it helped me with my climbs.
I no longer spin simply because I didn't renew my gym membership given that I run more than anything else.