Picked up the new Seidio 3200 extended battery for my GS2 and I love it! I also had the ext batt for my Captivate but this battery is much thinner! Don't know how they managed it but the thinness sure is appreciated. Amazing battery life with my average usage, no power user for sure and radio's only on when I want them on. Also the "soft touch coating" that Seidio uses is so nice, you really have more grip. Door fits very well, tightly all around. You seat in on the bottom first, then work clockwise around the phone until it is all snapped into place.
Link to Seidio site: seidioonline.com
Link to where I purchased a tad cheaper: Seidio Innocell 3200mAh Extended Battery for AT&T Galaxy S II - Android Batteries - ShopAndroid
A couple pics:
Link to Seidio site: seidioonline.com
Link to where I purchased a tad cheaper: Seidio Innocell 3200mAh Extended Battery for AT&T Galaxy S II - Android Batteries - ShopAndroid
A couple pics: