The battery on my mid-2009 MacBook Pro was well past needing to be replaced. It was only holding enough of a charge for 1\2 hour if that much.
The replacement took me only about 10 mins since I had been "under the hood" of my laptop on several other occasions regarding upgrades.
This is not a difficult replacement for most folks if they have done any upgrades before. There are 10 screws on the bottom cover of the laptop and care needs to be taken to ensure they go back in the same way they came out using whatever method(s) work best for you. The only screws holding the battery in have a tri-wing, or tri-star, pattern and using the Tri-wing Y1 Screwdriver offered through iFixit is a must to ensure you can easily and correctly remove and replace battery. After removing screws a simple unclipping the ribbon cable connecting the battery to the logic board and lifting battery out is all it takes. Installation is in reverse. Work carefully and methodically and all will be fine.