I'm one of the few nutjobs looking forward to the supposed 3500mah brick hahaha. "wahh it makes the phone too thick" well if you want to be able to use a phone with the screen on for 10 hours straight guess what lol
I've had three of Seidio's slim extended batteries and they're decent. The biggest problem with them is that they don't have the longest life over time. They seem to lose their charge quicker than the stock Moto ones.
I'm waiting to see if the official Verizon/Moto battery really adds much thickness and if it will fit in the current VZW hard cases. If it doesn't, then I'll go Seidio.
I've purchased extended batteries from Seidio before and Stang68 is right, they're great for about six to eight months, and then they can't seem to hold a charge for more than half a day. When they die, they die fast. Either way, I'm getting an extended battery from somewhere. I do events for a living and I need something to last at least 14-16 hours a day.