Thats me just installed my 4800mAh battery and its fantastic. Normally within 30minutes ill loose circa 30percent of my charge but this baby's only gone down by 1percent in the same time.
Size wise it does make the phone a bit wider, its now 1.5cm wide at the widest point (or thickest, same thing).
The extended back isnt square and angled like the one for the s2, its a nice smooth, curved white and feels lovely to hold. Fits in my hand perfectly and I have very small hands.
I'll get a photo up later using hubby's phone when he finishes work. I'll get photos of the comparison between battery sizes and likewise the overall width with the new back panel on.