I've had one of these cheapo extended eBay batteries for about a month now (eBay item number: 141212430394). The battery itself is great. Heck, it's almost a necessity with some of the newer unoptimized ROMs (Albinoman's 4.4.2, especially). It's nice being able to keep the stock battery in my wallet, but I am afraid to switch it, when I run dead. Why?
The battery door/back cover is crap. It's flimsy plastic and the hooks the hold it on are stupidly prone to breaking off. The top two hooks broke off within a week. Thankfully, the cover still stays on. Also, it makes it impossible to use a normal Vivid case, unless you cut a hole in the back of it.
Still, for 8 bucks, free shipping, I can hardly complain.