Bought a refurbished MacBook Pro off of Amazon for my son to take to college. Periodically it would go into low power mode even when the power cord was plugged in. The battery would indicate it wasn't plugged in sometimes. Jiggle the thing and it would be ok. Opened it up to verify the connection was solid and decided to try a new battery and cross my fingers since there is no warranty on the computer.
Ridiculously easy to replace the battery in these things, well that is as long as you have the right Tri-point screwdriver, which I do in my iFixit 64 Bit driver kit!
Why replace when you can repair, save a ton of money, and get great satisfaction when all your friends look upon you with awe at your repair prowess?!