I bought a Yoobao 2200mAh battery from Amazon about a month ago (http://www.amazon.com/2200-Extended-...0571465&sr=8-6
). Battery life is DEFINATELY improved. I've done no truly scientific measurement of how much better the battery is, but I will say I can now use the phone how I see fit (with the exception of my email, which is not set to download automatically for various reasons) without worrying much about charging the battery every time I blink. It does indeed add extra weight, but nothing terrible.
Bottom line...even though the battery meter is no longer accurate, it doesn't matter. I don't have to charge the phone after playing a game and listening to music and surfing a bit. That in itself is worth the $24.99 that the battery cost.
The way the extended back cover feels is pretty nice. It gives it a kind of grip feel...but that grip stuff is also starting to peel from the corners, so the build quality of the new battery back isn't all that great.