I added a grip to my D700 and right away I noticed what a great advantage there was going to be with the balance.....in fact I liked it so much I went right down and bought one for my D300 also. Since I bought the grip, I've had at least three experiences where I was in the middle of shooting wildlife when my battery changed over seamlessly. I was very pleased that I did not have to stop and put another battery in the camera. I would have missed some very nice shots had that happened. Especially in the winter, your batteries will go down fast and it's no fun to change batteries in the cold.
I tried shooting with it the other day without the grip and it was so nose-heavy it felt like it was going to tip out of my hand. This is especially true when shooting offhand with a big lens.