I own the Canon brand grips -- not 3rd party. I have come across a number of people on the Canon forum (this has probably happened three or four times in the last couple of years) where someone thought they had a defective camera body and after the dust cleared, it turns out the camera worked perfectly... if only they removed the 3rd party grip. I've heard of battery drain issues, but one of the more common issues is that the shutter button will refuse to work on the main camera when the grip is attached.
Obviously this doesn't happen to everyone or we'd see a LOT more threads about not buying 3rd party grips. But I've _never_ heard of a problem from anyone who used the factory grip.
I've hard that as grips go, the Vello grip seems to be pretty reliable -- but this is second hand because I don't actually own a Vello grip.