I had the chance to use my brother's Canon grip on a 400d for a few weeks before deciding on one; it made the camera feel much nicer in the hand, better balanced with long lenses and just more substantial.
As I'm a cheapskate when I bought one I went for a Phottix one from ebay, feels just as good as the Canon jobbie works perfectly and cost about a third of the price with a couple of spare batteries thrown in. You do hear people say don't touch the third party ones but I'm convinced this is based on hearsay and rumour rather than direct experience of 'em, so I'd say go for it and save some cash.
HTH,
Stu.