I wanted to maximize the amount of space dedicated to both OS's running on my computer, Mac OSX and Win7. (Bootcamp takes space away from the partition and I wanted each OS to have equal storage space.) The internal drive would be my Windows drive (Windows has to be installed on an internal) and the new Optical drive would be my Mac. 2 500GB hard drives, 2 operating systems, 1 Macbook Pro.
The installation went quickly and flawlessly. The iFixit guide was superb and easy to follow.
I felt incredibly empowered. This was my first time really getting my hands dirty (the only other time I've opened up a computer was to install RAM.)