I bought a Canon grip for my 60D, and within a week it broke.
Had a hell of a time tryingot get the damed thing off the camera!
So, I then bought a Zeikos grip for a third of the price of the Canon, and it has worked perfectly for the last 18 months or so.
I'll be getting my 5D3 in a month or so, and I'll be buying another Zeikos grip for it, whenever they come out.