I have 4 third party grips, one for my 40D, 2 for my 5DII, and 1 for my 5DIII.
All worked well.
I do have two observations:
First, the wheel at the back of the camera stopped responding when the grip is turned on. Turning it off returned the wheel functionality.
Second, the battery seems to drain faster when the grip is turned on. Probably because of the internal electronics in the grip.
On the 5DII grips, I took one apart so I can modify the electronics to accept a remote shutter release receiver so I can reduce the external wiring for the remote. But, with the complex electronics modules in the grip, it just became too daunting a job. But what I found out through the process is that on the top mount where the 1/4-20 screw going into the camera tripod mount, there is a metal plate the size of the grip! The same is true of the bottom of the grip. So, there are 2 plates within the grip, securely screwed to the shell. It is satisfying to know that the structure is very secured in terms of construction. There is no need to worry of the camera breaking off from the grip.
Know this, I feel confident about using the grips.