Read some reviews on XDA and received my first extended battery for S2 with a free cover ( the original battery cover wont work with the new mpj battery) last week. It actually is supposed double the thickness and yes, the stock cover can not fits with the new battery at all. The black cover fits with the battery properly and it is slightly bigger, offering enough space for the battery to expand when it warms up. About the performance, the extended battery definitely brings a great difference of battery life. Personally I don