I bought the Seidio Extended battery and it is amazing. Charging used to be a daily thought but now I can go 3-4 days. It almost surprises you when it needs to be charged. I am a modest user and usually keep my radios off until I want them on. I had a Seidio ext batt on my Captivate and it was huge. Somehow on the SG2 they kept the size thin (for an ext.) and like the Captivate batt, still has the soft touch coating on the back cover. The cover fits tightly and snaps in place easily.
I ordered a 3800mAh battery. It came with a custom back cover - all for $11.39, which included shipping from Hong Kong!
It works great. I am a light user - can go two days without a charge easy. Plus, I like the bump out on the back, as it makes it easier to grab the phone from my pocket and bring it out right-ride up, making it easier to locate buttons.