I debated about getting the Canon grip for the 5D III. I finally did get it because the price was right at $240. However, it sits in the bag and doesn't get used much.
A 1-Series body feels entirely different in my hands than a gripped camera. The one good thing about the 5D III grip is that it is the closest to the feel of a 1-series body.
If the 5D III grip price wasn't right, I would have passed.