Having owned my Galaxy S2 for over four months, my only gripe had been with the poor battery life.
I was finding that I would take my phone off charge at 7am (100%) and by 10pm it would be nearly dead (<12%). Roughly 15 hours battery from a day of normal use. (A day of normal use being: 3G and WiFi on most of the day, few texts and phone calls and about an hours gaming and web browsing with screen brightness at max.
The extended 2000mAh battery and cover that arrived looked to be of the usual high quality offering that you would expect from Samsung. The back cover is identical in appearance to the original cover but it is slightly thicker. In my opinion this is a good thing; With the 2000mAh battery in and the new back on, the phone feels more sturdy and sits in the hand better. It feels more like a serious phone, rather than a fashion accessory.
I have a generic silicone case from eBay and this fits over the phone with the 2000mAh battery and cover comfortably with only a small ridge at the bottom where the case followed the contours of the slimmer battery. All in all, it's a fit I can live with.
I left the battery to charge fully with the phone turned off and then used my phone as normal. On the first charge I got about 30 hours out of the phone with arguably slighly lighter than normal use. I let the battery drain till the phone turned off then fully charged again. Second charge I got 15 hours of normal use and still had 40% charge left in the battery. Third charge is currently at 70% after 9 hours of normal use equating to roughly 30 hours in total.
Overall this battery is a worthy upgrade. The phone looks better with the flush back, it feels sturdier with a bit more weight behind it and ultimately the battery life of the phone has pretty much doubled! This is the battery and back cover this phone should have come with and you won't be dissapointed with the benefits of the 2000mAh battery vs the standard one.