Okay, I purchased a 3500mah from ebay, just to see if it would be too thick for my liking. It only cost $10 and since the 2000mah was pretty much a fail (in fact, the 1800 mah I purchased was 10 X's better), I figured I'd give it a shot for days I know I wont be able to get to a charger.
Results, since I took it off the charger at 730am. I listened to music on the way to work (30-40 minute drive) w/o it being plugged in. There are also certain areas where signal drops out.
Looked at a few emails, sent a couple texts. Battery is still at 82 percent. Not bad.
I really do like the functionality of this battery. Even though the size is more manageable than I thought it'd be, I can't see myself using this on the regular (only for days when I know I can't get to a charger). All-in-all, I think it's worth having for certain purposes.
Just thought I'd share my thoughts on the 3500mah battery from ebay.
Oh, and btw, for those of you that don't want to add a case, I actually managed to install a custom skin that I had made and even though some countours of the case didn't fit quite right, it still works perfect for my needs.