My neighbors old unibody macbook was getting really slow, and the 160gb hard drive it came with was so full they kept having to move things off to an external drive just to keep the computer working.
It went great, the guide was great. I found the comments about not removing the subwoofer cable very helpful, and did as was suggested and skipped that step. There is plenty of room to remove the optical drive without removing the subwoofer cable.
The only place I ran into an issue was when I was installing the hard drive enclosure. I used the original 160gb hard drive and it was just a little thicker then I felt the optical bay could handle, I had to apply some light pressure to get the screws to seat correctly.
I installed a new SSD in the factory hard drive space, vastly improving the performance of the computer. Since I reused the original hard drive in the optical bay, I didn't run into any issues with the speed of the drive as some have reported.
I found it hard to find a drive that was clearly just SATA II (3gbps), so ended up reusing the original hard drive that came with the laptop. I had hoped to put something bigger in, but I wasn't comfortable with the level of information I could find on larger drives and their compatibility with the slower optical bay connections. Since I was doing this for a friend, I wanted it to be bullet proof.