It takes some practice to get used to it. At first until you learn to control it, and build up the muscles in your wrist or wherever you will be kind of out of control on bumps and stuff. It is great for long distance because your thumb doesn't get tired. Practically everyone in the desert runs twist throttles. And, most everyone in cross country and all that stuff use a twist throttle. You might not see quite as many people using it on motocross though.
a problem that I have seen (not experienced) first hand is if the quad rolls over onto the handlebar it can pin the twist throttle wide open with your wheels spinning wildly in the air. buddy of mine put one on his 300ex and while traversing over pile of logs (what an idiot) the logs slipped and then he slipped, the thing rolled over and stuck with throttle open, he was ok.
Hmm, never heard of that happening before. During V2R I rolled ours over, and it didn't stick during the rollover, but it did stick a few miles down the course. We think it was just because it got dirt everywhere, after messing with it and WD40'ing it it was fine for the rest of the race.