Having a user replaceable battery means you can purchase a replacement and swap it out without the need for a Genius Bar appointment or involvement of Service provider. Wether you are in or out of warranty, we do suggest making a Genius Bar appointment, or if you don't have an Apple Store available in your area, look for an Authorized Service Provider. They will get you an original Apple branded battery usually at a lower price as they get the old battery from you. Buying a new one off the shelf is always an option (for supported models only). As long as you can get an Apple branded replacement battery try to stay away from 3rd party battery manufacturers. Their batteries, although compatible, usually have a much shorter life span and in some cases can even damage your machine. They're sometimes much cheaper priced and tempt a lot of people but you will probably soon find out why it was that cheap. For Vintage and Obsolete products the only choice will be 3rd party manufacturers as Apple or Apple Service Providers will no longer supply the replacement part. More Info on battery replacement here. MacBook Pro specific information here. MacBook Pro how to information here. MacBook how to information here. PowerBook G4 how to information here.