Like everything else about bikes, what is salvation for some is poison for others. My HardRock had gripshifters, which seemed to really cause a flare up of ulnar nerve problems (hand & finger numbness that lasted long after the ride). I have not had any problems since I gave that bike to a neighbor kid and got a RockHopper with Shimano shifters. For me I think the problem was the thickness of the twistgrip was pushing more weight onto the base of my hand, or it was just the torque involved in shifting.

I don't know how we are supposed to get around these problems, since my problems didn't start until after I had been riding the bike for a week, which is a little longer than most shops allow for a test ride. Most rental bikes are not new models, and often don't come in a full range of sizes.

Cycling is most definately a NOT a "one size or style fits all" activity.