Some come loose, and lose battery power. Some grips fit perfectly, but have a battery meter problem, where the camera shows low battery levels after only about 100-200 shots, then shuts off the camera, while it should really last for more than 1,000 shots.
And others say with heavy lenses, in portrait mode, especially with a flash, the grip would just lose contact, and all power is immediately lost.
I believe Canon has a fix, if you just send it in, but some have done so, and still report the battery meter problem. Then again, there ARE some good grips out there that work properly in the first place.