When my 1DmkII went belly up, i decided to get a 50D and grip it, i didnt think it would feel right without one. I was right, i use it a lot but what i will say is that it doesnt feel anywhere near as nice as the 1D portrait grip. The 1D feels comfortable in the hand, the grip feels like it just needs to be there.
I will always grip my cameras now because i like the feel, plus my wrists are a little crap and twisting them all the time isnt comfortable after a while. What i do like about the grip though is that if i go on holiday, i can take it off and have a lighter walk about camera then i did with the 1D. So, really, its a big thing for me, but i can totally see that if you have never had a "full sized" body, then a grip feels odd and especially as the grips arent as nice as the moulded bodies
Mike