I bought a non branded grip for my 5DII a while ago and sent it back after only a week, it looks exactly the same as the Canon version, but doesn't quite function as good. The area on the bottom where you would screw in a tripod mount had no "beef" to it and felt really thin, so after screwing on a plate and clicking it into the tripod, it wobbled considerably. Also the grip couldn't communicate battery info very well, and even though I was using a freshly charge battery, the camera would think it was nearly flat and not work after only an hours use. Also the vertical shutter button felt bit "clicky" compared with the smooth action of the 5DII's.
Not saying they are all like this, but my one went back straight away and I have missed using a vertical grip since upgrading to the 5DII