It's damn good (350 more mAh). Makes me think the whole 'thin phone' contest between manufacturers is unnecessary and at our expense. There's no point having a phone that's 8mm thin if the battery life isn't that good, it's like having a fast compact sports car with a 20 litre fuel tank (roughly 1 gallon).
The extended battery makes it how it should have been, now the phone has 1 profile top to bottom, no butte, and the camera gets a bit of protection.
It's also good having a spare battery in the original 1650mAh one.