In reply to Oujmik:
> Constant changes of pace, position, cadence, gear... just annoying
A pure road cyclist would find that annoying, but as 50% mountain biker, I didn't - although one instructor does a very annoying stand up/sit down bit which I just ignore.
> Instructor shouting inane things such as "there's a pub a the top of this hill!" - I don't want to go to a sodding pub., I want to ride my bike.
I stopped going to one class, as the instructor was a bit of a prat and did this sort of thing, but other ones are more subtle about it.
> For a while, I experimented with just training by myself on a spinning bike in the gym. This resolved some of the issues, but not all. It was vaguely effective as training I think, but I'm not doing it anymore because it was dull and because I'm not really training right now.
I tried that and found it mind-bogglingly boring, too - I preferred the classes, as I felt I got more out of it. Mate of mine bought a spin bike from eBay and has it in his garage - no idea how much he actually uses it, though.