The original battery was rapidly loosing its capacity, falling below 20% of its original specs. This left my Macbook Pro with less than 1 hour of run time on battery (and much less if actually using it for anything).
Easier and faster than taking it to the Apple Store, from ordering the battery to swapping it out. Shipping was extremely quick, and had the battery swapped with the laptop booted back up in less than 5 minutes. I've swapped hard drives and upgraded memory in the past, so I knew what to expect getting to the battery...
Don't forget the tri-star driver - not a common tool many people will have laying around, nor is it easy to find in most locations.