This is the first time I have tried a battery grip with a DSLR, and I am already hooked. It is a really nice addition to a camera that is already ergonomically very pleasing. You might think that the grip would somehow unbalance the camera body, or that it would be unwieldy or perhaps a little wobbly, but in reality it fits like a glove and has a reassuring solid feel. It undeniably (and obviously) adds to the weight and the heft of the 7D, but personally I like this and I think that, in my hands, the result is a better balanced and more easily manoeuverable camera. The difference when shooting in portrait format is obvious -- the camera is just much more steady than it used to be. I also found that when shooting with a small lens (e.g. the 50mm f/1.8 II), the grip enables you to steady yourself much more effectively than just by grabbing the lens alone, since it provides an extra 'ledge' on which to put your wrist (or even your hand).
The grip itself is nicely designed. It is not ergonomically identical to the grip on the body itself -- it is much more square, but the shutter button and wheel are cleverly located to make them fall just under your fingers where they need to be. The various buttons and controls don't feel identical to those on the body, but the difference is not huge. It is true that, as many reviews state, the shutter button seems to be more sensitive than the one on the camera body -- I keep taking extra shots by accident. I daresay I'll get used to it. Occasionally I just nudge the shutter on the grip even when I've got the camera in landscape orientation -- particularly if I'm reviewing images on the screen -- and this causes the camera to focus and meter, which can be puzzling and annoying until you realise what's going on. This is presumably why you can turn the controls on the grip off. Other than this, the controls work well and they tend to be where you expect them to be; I have quite large hands so can still operate the quick dial on the back of the camera 'as normal' with my thumb as if I were holding the camera in landscape orientation. This is really handy, since this dial controls so much (e.g. aperture in M mode, focal point selection, etc.). The only thing that *is* different from 'normal' is my inability to reach the joystick multi-directional controller. If you are used to selecting focal points with it using your thumb, you may find this a slight disadvantage -- although really it is a pretty minor one.
The battery info function on the camera works well with the grip -- you can keep track of both batteries in the grip (if you're using two lp-e6 batteries). You can also use AA batteries, although I haven't tried this yet.
So I think it is a very worthwhile addition to a great camera. One significant disadvantage is its price, which in my opinion is about 50% higher than it might otherwise be, but this just seems to be a fact of life with Canon's accessories!