My experience is that any generic battery for any product performs as good as or better than the OEM at first, but loses the ability to hold a charge after a short time. At that point a double generic isn't much better than a stock original.
I STRONGLY recommend skipping the generic/third party stuff and going straight for a spare OEM. Swapping out is easy, and you won't have the extra bulk.
(This coming from a guy with two 1500mah generics, two 3500mah generics, and two 1500mah Samsungs.)
FYI: When a someone says "nandroid" what they're referring to is CWM or ClockWork. I wish someone had told me that months ago.