You didn't say what camera you have, but it has to be a 5d or lower. The 5d grip is uncomfortable and designed by a local bricklayers union, but it works. Before I'd buy any strange appendage off ebay, I'd want to try it out and I'm not sure you can do that. I'd buy Canon, but that's just my opinion.
zzt... hey, whats that smell? It;s my camera, my crappy chinese grip is on fire.
No seriously, people her have used them. But I woudnt bother - they usually use some weird battery tray that looks pretty flimsy. Maybe a used one on ebay?