I purchased the extended battery for my NS4G from gorilla gadgets. Only 27 bucks, but you get what you pay for. Battery door doesn't fit well, it's huge, and I'm noticing very little difference in battery life. Customer service has been rude and not very helpful. FYI.
I been using a 3200MHa battery from gorilla gadgets in my droid charge for a few months and it was money will spent. I personally don't have any complaints with the battery door and I find it easier to get off when I need to then the stock cover. The other thing I like is that the stock battery cover is too slick and I find that it sometimes slips out of my fingers. The extended it one doesn't have this problem. But keep in mind it does add weight and thinness to your device. Personally I like but some people may not like the added weight.
Yes most people will find that the battery doors that are aftermarket are not going to fit. Even Verizon extended battery cover doesn't fit..Luckily my original one fits with the extended battery mad by Samsung for the Verizon brand .