My PRS-500 is coming up on 2 years old, and the battery had started to get a bit flakey - I've really abused this battery, alternately leaving it totally drained for weeks, or leaving it on the charger for weeks. Anywho, I got a replacement battery from:
http://www.batteryship.com/htmlos/ht...Reader+PRS-500
and did the swap. Batteryship included all the necessary tools, which was nice. Prying off the silver plastic side pieces was the only tough part, just be gentle, and don't forget the two screws around the SD slot. Then it was just a matter of taking out all the screws on the sides and bottom, pulling it apart and swapping the battery.
The original battery is glued/taped in place, so it takes a bit of prying. You can remove the whole battery tray with two screws if necessary.
Anyways, it was a simple process, less than 20 minutes. New battery is now fully charged and I just flipped through 150 pages (in quick succession) and still have 4 bars. With the old battery, 20 rapid page turns would have me down to 2 bars.
Anyways, hope it's helpful. Definitely nothing to be scared of.