Easier than the old "smack it until it works"
I'm an IT Manager, and I carry a set of iFixit tools around with me everywhere I go. I have a late 2009 MacBook Pro that is well maintained and serves as my daily driver. I've replaced just about everything that is replaceable except for the battery, which prior to performing this replacement only held 40% of its original capacity. This gave me a little less than an hour of real battery life—definitely not enough for my needs and left me chained to jumping from outlet to outlet.
Very easy—simply follow the guide and you'll be on your way within 10 minutes.
1. Although the guide calls for a pentalobe screwdriver, it isn't needed. It is useful, but a small flathead will work. Just be careful not to strip them.
2. There are a lot of screws—simply remember where the pentalobe screws go and where the three long screws go, and you're golden.
3. Don't strip the threads. I see this a lot in my profession when a screw is screwed in without first lining up the threads. To line up the threads—while putting in the screw, turn COUNTERCLOCKWISE until you feel or hear the screw "click" into place, before screwing in.