My battery, which I had for 2 and 3/4 years was down to about 50% of its original life. The erratic behaviour of the battery suggested that it was defective. Not having purchased Apple Care, I wanted to fix the problem myself.
Went smooth as butter. I'm a seasoned computer repair guy (for family, friends, and my own amusement). I also upgraded my Macbook's ram not that long ago, so I was familier with the internals. The battery itself installed perfectly. It took maybe 15 minutes to do the job, not including the time I spent cleaning the inside of the machine - getting rid of unwanted dust and dirt. I'm not a big fan of the Macbook Unibody's non-slip bottom, as the rubber edging is prone to coming loose; I've had to tape/glue it a couple of times already.
I can't think of anything too important. Just make sure you have a small dish (that doesn't tip over easily) for each type of screw or part. Also, make sure you work on a surface that's not too low, as longer jobs while bending over (especially for a tall person like myself) can anger your lower back.