My old battery was nearing death. Couldn't hold a charge for more than 2 hours unplugged. Of course the Apple peoples wanted $150 for the battery and another $100 for labor since my MBP is far out of AppleCare coverage time. Plus, they wanted to keep it for 4-5 days. They of course strongly discourage do it yourself repairs. But with something this simple? Go for it.
The repair is very simple and fast. If you can twist a screwdriver, you can do this one. I was done in under 10 minutes. The new batteries sold here on ifixit.com are great. They really are new, OEM batteries that show a zero cycle count right out of the box.
Just go slow and get a small container to put the screws in while you are working. They are super tiny and will surely be lost forever if dropped. Also, use just enough pressure when loosening/tightening the screws. They are fragile little things and will shear very easily!