I've purchased a few third party grips over the years with varied degrees of satisfaction experienced. The last third-party grip I purchased seemed to work fine. However, when attached to the 5D III, it caused the 5D III's multi-controller joystick to malfunction. Note that I didn't say the battery grip's multi-controller joystick malfunctioned – but it caused an issue with the camera's own joystick. I attached a BG-E11 grip and everything worked fine.

I know the OEM battery grips are expensive, but I've come to the conclusion that they're worth the extra money compared to less expensive third-party ones.