The OEM 2000mAh battery is highly sought after because it deliers more than 20% more battery life while still maintaining the Galaxy S II’s thin physique:
We got around two days worth of pretty heavy usage with our Galaxy S II thanks to the beefed up battery power; in the era where we are resigned to charging our phones every night, this offers a very welcome respite. With a less intense usage (ie; something more akin to regular use), the bigger battery lasted quite a few more hours. The extra 20% of capacity certainly gave our S II plenty more juice.
Hope this helps guys!