I have a BG-E2N on my Canon 40D. It's tight, fits well, holds AA's and well as a pair of BP-511's. I haven't pushed the limits of batteries yet - I haven't had but one little practice portrait assignment, so I'm out on usefulness and length, although it seems to stand to reason there.
I didn't buy the grip for the batteries, though: I bought it for the grip. I used to shoot a 1N, and it was a monster. I sold it and started shooting my A2 backup body, and it was - for an already large body - too "small feeling". I have dinky-ass little hands, and little cameras, oddly, feel funny. I bought a vertical grip (no battery) for my A2, and I was happy.
I Ebayed mine. It was pricey regardless - brand new from some guy in the midwest it was $140, with all parts, unregistered warranty card, the AA tray, and shipping. It was an admittedly frivolous buy, given my level of work and the fact I would be happy even if it didn't take extra batteries, but I'll live with that, and the fact I'm more comfortable shooting with it, especially in portrait with the second button and dial.
One note: the AA battery tray is a VERY cool feature, but I had heard it was a little flimsy. Once I got mine I checked it out and...it's a little flimsy. I'm afraid to put it in my bag without some kind of case, because I'm certain I'll take it back out in little bits.
Cheers!