Usually I'm used to use the smartphone with many apps in the background (WhatsApp, Dropbox, Beautiful widget and many other) and i often run games, especially in down time of the day. Im also linked to a carrier that has only 3G. Before attempting the battery I used to utilize three batteries so i can change eachother when the current one is downloaded.
With the Mugen's 4000 mAh battery i have found no problems to arrive safely at the end of the day, even when I spend hours playing.
In addition, the cover included in the package greatly improves the grip (im serius!).
I've tried it for 5-6 days but if you want to get more details, statistics and screenshots you can view this video review (in Italian)
You can buy it from www.mugen-power-batteries.com