I was going to link, but there are a ton of posts about batteries... So it will probably be easier for you do 3 searches of this forum for 3500, 2150, and 1750, instead of coming back to this post to keep clicking the links. You'll get several posts where there are plenty of reviews of the extended batteries and you can pick out the info you need.
Also, you're going to have to decide what size battery you'll need. If you're a power user, the larger batteries, 2150 and 3500, will probably be the way to go. If you're a light user, the 1750 or another stock (if you don't mind switching batteries) might be fine.
I also can't speak for any other batteries except the OEM from HTC 2150 and the Seidio 3500, but the majority of the people here seem to stick with OEM's or Seidio... So, enjoy the research, and take your time. Pick what will work best for you and not what "everyone else" is doing...