Battery grips seem very expensive for what they are. Its ~£100 for the Canon branded one, or half that for a 3rd party one with a few extras like remote control included.
Is it worth paying extra for the Canon product? (Camera is a 500D if that helps)
i got mine on ebay direct from china. has a built in battery 4 times that of the normal camera ones. Only has a additional shutter button, but thats the main reason i use it.
If using a 3rd party grip ensure that it has the master dial and other functions built in or else there's not much point.. I have a canon BG E2N and got it on the bay for £85 and well worth the money.
I have used the Phottix one for D300s for ages at about £50 vs over £200 for the Nikon one its a no brainer. Never off the camera - top quality and faultless. So no, the own brand one isn't worth it. I would have to buy 4 replacements before I broke even.