Hi,

I purchased this item from: spiderfoot (http://www.spider-foot.com/batteryusb-sync-dock-cradle-charger-kit-for-samsung-gti9100-galaxy-s-ii-910c5e-p-928.html)
http://img23.imageshack.us/img23/8107/spiderfootbatterycharge.th.jpg (http://img23.imageshack.us/i/spiderfootbatterycharge.jpg/)

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I haven't tested it thoroughly, however i can say that it works, the battery arrived with around 60% charge, the spare battery charger is powered by the mains adapter, the phone can be charged simultaneously only if you connect the usb cable to the cradle. The phone connects to the usb connector with ease.
It was relatively cheap (around 17€), and took about 2 weeks to arrive with free postage.
There is a screen protector and a stylus included, that i won't use.
Will try to test the battery a little more in the next few days, however i don't even have a good feel for how long the stock battery lasts, so my comparisons to it will be ill informed, at best.
It seems a reasonably well built product, sincerely hope it will stand the test of time.
Cheers

I tested the battery yesterday, it only charged to 97% however it lasted for 18 hours of moderate use except for 2 hours of fm radio while riding my bike.
It seems to not last as long as the original battery, but the difference is not very big. While idling through the night with wifi on and a few widgets and mail updating it lost around 1% per hour.
I always kept wifi on, but most of the time was only on 2g.
Cheers