If you want a good, reliable extended battery, get one from Mugen Power.
I've been using it for half a year now, and very satisfied.

2400 mAh, very long standby and more than a full day of extensive use (many hours of continuous browsing, music, whatever). I've never had it go dead on me during the day yet.

It DOES make your phone quite bulky (thickness increase of around 50%), and it doesn't have a lens cover in the back - however the lens is very deeply recessed and I've yet to get a scratch of any kind on it. Just dust.

Here's the link:

http://www.mugenpowerbatteries.info/...cts/HLI-N900XL