I got the HTC 2150 DInc battery about a month ago. It increased the thickness to equal the Mot Droid 1, but since the thickness is not increased at the edges it still feels fine in the pocket. Also same thickness as my iPhone 3g with a case.
Stats:
I keep GPS, wifi & bluetooth off and only turn on when needed. I don't use a task killer.
Stock battery - charge overnight, unplug at 6am. By 2pm it is at 30%, which is my cutoff for recharging. So stock battery gave me 8 hrs of use.
Official HTC 2150 battery - charge overnight, unplug at 6am. At 11am on the *following* day it is at 30%. So 2150 gave me 29 hrs of use!
I have had this battery for about a month and this is consistent battery life. If I use it more it drains more, conversely I have had it go 37 hrs without having to charge it.
Oddly enough, recharge time is the same as the original battery. Overall, can't recommend this enough. I got it because Amazon had a deal where it was $30 so it was worth a shot.