It depends. If you want to do the occasional spin class, go do the class and have fun. Use it to replace your weekly intervals ride in your program. However, if you want to do a spin class every week, you should keep an eye on your intensity. If doing a weekly spin class will keep you happy and help you be consistent, then it is worth it, but just try to keep your intensity mostly in zone 3 (tempo) or below for 75% of the workout (feel free to go nuts for the other 25%) during the prep phase and early base phase.