I have an older Trek MTB which has grip-shifters on it, and I really don't have any complaints about them. I love being able to shift thru most/all the gears in one single motion, rather than click-click-click-click-click-click-click. I've been test-riding new MTBs with both SRAM and Shimano (I like the double-motion top shift lever on the latter), but why don't they make grip shifters anymore? I have heard that some complained about accidental shifting due to slipping onto the grip shift, but that's hardly ever happened to me.