I got a used 2007 Macbook Core 2 Duo a couple years ago. When I got it the SuperDrive wasn't reading discs, so one of the first things I did was replace it. However, as that era of SuperDrives were notoriously faulty, my new SuperDrive also failed, so bad that I had to pry a DVD out of it with tweezers. Left a nasty scratch but thankfully the disc still plays.
Anyway, a after having a useless drive for over a year, my HDD started to go out, so I decided to replace the optical drive while I replace the HDD.
I used ifixit's HD enclosure and life saving guide to install my new 750GB HD where my broken optical drive had been, and while I was at it I replaced my old HD with a new 90GB SSD. For daily tasks, my 2007 Macbook now feels lightning fast, especially compared to previous performance. Very happy to be making use of all the space in the laptop after having a dead drive for so long.
The repair went super smoothly. Thanks to ifixit's guides I am now fairly familiar with the inside of my Macbook. Best of all I didn't lose any screws this time!
Best way to speed up an old MacBook without losing storage space: Put a SSD where the HD was and a HD where the optical drive was.