I will have had my Seidio 4000mah battery for a full week on Friday, and I hae zero issues whatsoever. Althouh admittedly I do keep it charged constantly, whenever I do put it to use, it's always reliable for the usage that I put it through. I'm a heavy gamer and movie watcher on a daily basis, and I have never experienced any power shortages or shortcomings. However each chance I get, I am always challenging this battery to see how much I can beat it up, with the amount of daily use I can get out of it.
As the previous poster elluded to, it does make the phone rather bulky, but if you're not into conformity and stealthiness, then you'll be fine. It's size also doubles as a camera protector too, because of the amount of depth between the lenses and how much the battery case petrudes. It's not that bad to me, but some people just don't like bulky phone. And if you're not into using screen protectors on your lenses in fear of them distorting your images over time, then this battery case is perfect.
My advise, get it and see if you like it. And if you don't see an improvement over the stock battery life, then sell it.