Well after having a grip on my 300D and my 20D, I had sort of decided that I wouldn't bother with one on my 5d.
The extra functionality of having another jog dial and shutter release didn't seem worth it for the weight penalty. I'm never too far from A/C power, so the extended battery wasn't a factor.
What a mistake that turned out to be, shooting with the 70-200 f2.8 on was uncomfortable over a long period as the weight of the lens battled with the wrist muscles for control, shooting portrait was harder as your elbow pointed at the sky.
So while wandering down the high street I gave in and bought a grip for the 5, the familiar feeling I had with the 20D set up returned
although I'm sure the grip is bigger than that of the one fitted to the 20/30/40 series.
The camera looks massive, almost 1 series proportions
My advice for what it's worth, if you haven't already bought one.....do it now, you'll not feel short changed.