I bought the grip at the same time I ordered the camera. Ergonomically speaking, I like how the camera fits in my hand. Quite comfortable, but in my opinion, with the grip on, it fits even better. I double grip the camera so that my right hand cradles the bottom of the camera. The grip rests nicely on the palm while my fingers still can reach the lens focus/zoom rings. Without the grip, I'd be constantly crowding the lens with my left hand, moving the focus/zoom without intending to. The added bonus of two batteries, double the life, are a must for me, not to mention the convenience it brings when shooting in portrait mode.
It is hefty when you add the grip and battery, but the way I see it is my Canon AE-1P and 300MM lens with auto-winder plus 4 batteries weigh quite a bit too. Add the flash plus batteries for bonus grams
My only true gripe is hitting the grip buttons if I'm not careful, like when I'm previewing a photo and end up touching a button, making the preview disappear. I know I will stop doing this the more I play with it, so I'm not concerned.