Dead battery in a late-2009 (6,1) 13" white unibody MacBook. A do-it-yourself fix was much cheaper and less hassle than an Apple repair. And more fun!
Went smooth as glass. Used a generic replacement battery previously purchased on-line, and the iFixit instructions. The iFixit 54-piece tool kit that I ordered for the purpose (because it always pays to buy the best tools, especially ones that you just know will come in handy for future repair jobs) was precise, built to last, reasonably priced—and as a bonus the case that it comes in is thoughtfully designed to keep all the pieces nicely organized. The repair job itself took a grand total of 20 minutes, very straightforward.
Do it yourself! No matter what Apple tries to tell you about assemblies or parts that are not replaceable by users, always remember that authorized technicians are just people with good manual skills who are able to follow printed instructions. If you have those same qualities you can do just as good a job, and have the dual satisfaction of having done it yourself, and all the while saving a bundle of cash.