I've never used a grip before, but I decided to try one to help when talking shots in portrait mode. A couple of days after it arrived I got to use it at a kite surfing championships, and it was invaluable. With the grip fitted you can hold the camera and smoothly track fast moving targets in portrait orientation just as easily as you can in landscape. In addition, the extra weight provided by the grip does seem to balance the camera better. I like it so much I've decided to pretty much leave the grip on all the time - thankfully my 7D still fits in my holster bag with it fitted (it's snug, but it fits).
As others have mentioned, the grip shutter button is more sensitive than that on the camera. I'd say for sports shooting Canon have got it about right. Since the grip is thicker than the built in landscape grip on the 7D, somehow it feels right for the shutter button to be more sensitive. Can't really explain it, but it just feels right to me. Either way, you'll quickly adjust.
One major criticism (and it's valid for all Canon and 3rd party grips) is that the locking wheel is plastic and could easily get stripped of its hold on the locking nut if you're too "thorough" when you tighten it. A tip I saw on a forum is to put a tiny bit of high temp lubricant on the thread before first use, and accept that when the wheel stops moving with light to moderate finger pressure, then it's tight enough. Don't go Tarzan on it and it should be fine.