A friend of mine is a regular 'spinner' and she finally bought a real bike her first ever ride on the road was 47 miles, her 2nd was 67 miles - so the training clearly helps!
I hated them, I prefer the freedom of the open road, that and I hate paying for something I could get for free (a good cycle).
Yes spinning for anybody who hasn't done it before is 45 minutes of shear hell, I thought I was reasonably fit until I went to my first spinning class and quickly found out I wasn't but if you stick with it the strength, power, endurance and overall fitness improves dramatically. Its the quickest way I've ever found of getting fit and loosing weight.