i think its really a matter of personal preference. on the rebel series it is a must specially since it will improve the handling of the camera.
on the xxd series on the other hand the added weight just kills it. the 40d its ergonomic enough to be withouth a grip.
i once held a 5d gripped with a 24- 70 f2.8 L and a 580ex mk1 that was heavy!!! i cant imagine carrying that thing around. but i guess u get used to it over time
Ansel Adams used to climb all over Yosemite with a 60 pound camera, wooden tripod and a dozen glass plate negatives on his back...



