Check this battery out by Seidio. I got it when I ordered their Innocase and screen protector (both of which I really like).
I'd say that it does last longer than the stock battery by a noticeable amount, but not so much that I was incredibly impressed. I can definitely make it through the day with a few bars left to spare after being used fairly heavily (bluetooth on, wifi off usually). With the stock battery, I usually had to plug it in before the end of my night.
Also, there's no new back cover needed! Plus, now I have the stock battery as a spare if I need it.
http://www.seidioonline.com/product-p/basi17htp2.htm