I've used one from day one and can't really ride without it now. I've gotten so used to controlling the throttle with it that it's awkward to control it without one!
Up until recently, I used the wide one. It's a bit of a fuss to get exactly where it needs to be, but once you find the sweet spot it doesn't move. And on the stock grips, it left a small impression, so that if I removed it for some reason, I would know exactly where that sweet spot was when I put it back on.
I've been trying the skinny, longer one recently, since I put my new Rizoma grips on. It's been quite an adjustment, but it's getting better. I switched because the wide one didn't quite want to work with the Rizoma billet grips (even in the rubber grip area on the grips), and I liked that with the skinny one, you could just flip it up and over so if you didn't need it (or want to use it) it was out of the way.
The issue I've run into, at least with the new grips, is that because the skinny one doesn't velcro around the grip like the wide one, but yet uses pressure to stay in place, it doesn't always do so. Especially on a hot day, if the grips get hot they get kinda slick and it slowly wants to creep down lower and lower. Not a big deal really...you just give it a nudge and it's back up to where it needs to be. But a bit annoying.
I'd recommend one for sure, though. Like I said, I can't really ride without one now...lol.