Just like the size says, I get about 33-50% more life out of my phone than the stock battery.
This DOES fit in the stock backing piece, so modification or weird cases are not needed. The bottom clip however might not fully engage, which is no big deal (just squeaks a bit), and does not affect performance.
There is a deal for 2 batteries and a charger for $30 which I highly suggest. Even if you don't use both batteries in a day, its nice to have a spare just in case you forgot to charge up the phone or had a crazy day with constant usage.
If you are worried about battery life, the first thing you should look into is rooting your phone. I have found that ginger/sense roms have the best life and AOSP roms are undeniably the fastest. Between the 60 or so ROMs I have tried for this device, my favorites are MikRunny, CCM7 (V10), and Synergy (Warm Themed). The phone will run around 10-15C cooler and 20% faster.
Stock Battery/Stock ROM = 8Hr
Stock Battery/Synergy ROM (Most battery conservative @ 1.13ghz/128mhz smartass v2) = 12-15Hr
1900mAh Battery/Synergy ROM = 15-22Hr
1900mAh Battery/CCM7 (V10) = 12-19Hr