On another thread, I recently mentioned one unexpected advantage of using a grip: It makes it much easier to manual focus. It puts your left hand in a muh better position.
That may not apply to you, but if you DO shoot w/ MF a substantial amount, then this will be a significant factor.
I also agree with Danks & Chris that the ability to attach a wrist strap on the camera is a significant thing.
I couldn't care less about the extra battery, and I REALLY dislike the extra weight, but all the advantages far outweigh this negatives.
Had one on my 20d, really didn't see the point for me. Mostly I HATED the attention it grabs "ooooh, you must be a professional". It was a tad nicer to hold and I liked never having to think about batteries but of course was hugely bulky.
When I got the 5D I didn't even think of getting a grip. My ideal camera going forward would be a 1DsMkIII.....in a non built in grip format. I don't know why slr's attract so much attention but anything to attract less is a good thing - I still get way to much of it with my 5D if I have the 85L or 180L on.
