I have been shooting with my 5DmkIII now since the day it was available at the dealer. I shot it for a while without the BG-E11 grip because it was not available. The weight wasn't bad without the grip, then the grip came along. I screwed on the grip to the camera and holy heck did the weight increase! My boss shoots the 5DMKii with the grip and i can shoot that all day long no problems. My Rebel XT (yeah my first digital) and my 40D both had grips on them, i love the counterbalance that they provide; however, the BG-E11 is unbelievably heavy. I shot a wedding this last weekend, and after holding the camera in my hand all day (i dont trust a camera to be dangling at my side, i like the security of having complete control over where it goes and what it may come in contact with) my wrist was killing me. It still is a little sore, especially after shooting multiple weddings in a month.
I guess my thing is what was Canon thinking about making such a heavy grip??? It makes the camera uncomfortable to handle for a 16 hour wedding day.