My Canon 40D has the Canon grip while the XTi has the Targus grip (XTi sold maybe .. 6 months to a year ago)
When I had both cameras at the same time, I noticed the following.
- The Targus grip for my XTi fits very well with the camera.
- It can do everything that the OEM one can.
- The build quality is not as good as the Canon 40D grip, but it is not bad either.
- The leather like rubber feel as good as my Canon 40D grip.
- It did not experience a single communication problem with the camera.
However,
- The button feel (when pressed) is not as good as the Canon 40D grip. The buttons on the Canon 40D grip feel just like those on the camera, while the Targus XTi grip feel different. (It functions the same, just feel different)