I don't ride a mountain bike, but I do ride a comfort bike with grip shifters, and they work fine. I have also wanted to try other shifters, so I do have a set of Rapid Fire shifters I haven't mounted yet that I am waiting to try.
Now I need to add the LX shifters to my list of shifters to try... although I am a cheapskate, and will probably not buy a set, I would like to try a pair out if they were available on a bike I was interested in buying.