What I use is something like this:
5V 2A Mobile Power Supply USB Battery Charger 18650 Box for iPhone Cell Phone | eBay (there are heaps of different ones on Ebay)
This means that my cellphone is just as slim and light as ever, but I can charge my phone anywhere and any time. I can charge my phone in my tent overnight while camping - no problem using my phone to read a book in the evening while camping for a few days.
These portable charging boxes come without batteries and you can buy whatever 18650 (same size as inside of most laptop batteries) cells you like, I'll get some new Panasonic 3,400mAh cells at some point so that I have more juice in my charger. I will be able to recharge my phone from dead flat to full at least 4x.
I also use a ummm, one of these:
USB Data/Charging Extension Cable for Samsung P1000 - Black - Free Shipping - DealExtreme
This little device makes my phone think it is connected to a charger rather than a computer so it wont limit the charge rate to 0.5A.
It probably isn't vital but I sometimes want to get the phone charged up quicker.
So maybe someone can enlighten me as to the advantage of using an extended battery case as opposed to just taking a portable phone charger?