I used to carry around a couple of spare batteries for my HTC Thunderbolt, but it was a pain in the butt to remove the case (I always use a hard shell protector case for my phones), pry open the back and swap out the battery. With the external battery I don’t even have to turn off the phone.
Some people hate the plastics backs on a Samsung phone, I think they are great, pop right off and on, I can swap out a battery faster than I can find it in my bag.
How is the USB connector on your Thunderbolt? I had a Sensation and the little bar in the female side of the connector was made of plastic, I had to send it in twice to be fixed. I was going to make sure the my new HTC phone had a better quality connector but they dropped the removable battery and micro SD card from their new phones so I ended up with Samsung.