There's no denying the market for iPhone accessories is crowded, to put it mildly. But Third Rail Mobility recently introduced a slim case and smart battery for iPhone 4S that's worth your consideration.
Not only is it the thinnest battery-enabled case on the market, but the backup battery that comes with it is universal (it can literally charge 100s of other devices as well), hot-swappable, and stackable! Basically, this means that you can stack the batteries (sold separately) and charge everything from the micro-USB on your case.
The most important aspect to us is that the smart batteries are future proof. They will be compatible with all existing and future Third Rail cases, which means you'll likely be buying a new iPhone more often than you buy a new battery
The Third Rail Mobility system for iPhone 4S is available for 89.99 on their website, and you can read a recent hands-on review of the product here.