I just got the 2200mAh Samsung branded extended battery for my Mesmerize. I am currently on 37 hours off the charger and it still has 85% battery. This is going from ~15 hours tops on the stock battery. I'm very pleased with it so far. I even like the extra weight it adds (feels like an actual phone instead of a toy), and I'm kind of growing to like the way that the bulge makes it easier to hang on to.