This is the kit that takes the S2 from a great phone to a fantastic one.
Available from mobilefun but not exactly cheap (£24.95), contains a replacement 2000mAh battery and a replacement back cover for your S2.
Original battery is 1650mAh
Extended battery is 2000mAh
so its increased by +350mAh
which works out at
+21% (as a % of original size)
+17.5% (as a % of extended size)
which might not seem a lot, but its not an empirical increase in terms of actual battery life.
This is a genuine battery and none of your other 'nonbrandname' batteries you can get on ebay that are no better than the OE version.
Battery is slightly thicker and means that my OE mesh case did not fit.
So this meant I had to scour the forums to see what did....
some useful threads on xda ...
2000mAh battery thread on xda
2000mAh case compatibility thread
in the end I plumped for
SGP Samsung Galaxy S2 Case Neo Hybrid EX Series Soul Black
which has a gel inner casing that has a subsequent harder outer shell, and because its not a 'full' case the increased battery size (and cover) makes no difference.
I have already posted some battery stats but needless to say its a very worthwhile mod, that has increased my battery life to an easy full 'hard' days work, no longer I am concerned that the battery will run out mid afternoon.
commonly i'm at 40% last thing at night, and during low use times (weekends) i can often go a full 2 days!
Id say its added an easy +30% in battery life (maybe even more) , much more than the figures only suggest.
Get one :-)