I wanted more space on my computer for songs and files.
It was amazingly fast. I thought I would have to take a lot more of the components out of the computer, but I just had to unscrew the bottom plate, and then the hard drive, plug everything in and screw everything back on, and I was done! And the guide was the main reason it went so well. Very happy with the experience.
Definitely use SuperDuper to transfer all your data from your current hard drive to the one that you're installing. There is a video guide on how to transfer all your data on iFixit somewhere. Should be pretty easy to find.