I had upgraded the hd in my macbook but it was beginning to fill up and get bogged down causing the entire system to run very slow. I had read about ssd's and seen kits for swapping out the dvd drive for a second hd space. The ssd goes where the original hd was and the old hd goes where the optical drive was. Then the ssd gets used for the os and applications which allows everything to boot up and launch a whole lot faster.
The repair went great, I had already wasted money on another (cheaper) optical bay hd enclosure that was the wrong size and wrong connection so I had already used the ifixit guide to open my macbook for the surgery. When I got the optical bay enclosure from ifixit it took roughly 20 minutes to reopen my computer and install the thing. Truth be told I hadn't replaced all the screws from the first failed procedure, knowing that I'd be plunging into computer guts a second time.
Don't overfill your hard drive. But until the price of ssd's comes down for large capacities using a two hd system, one for os and apps and one for files, seems like a great way to go. So far at least.