Very nice! I've been toying with the idea on and off for a while now...the one thing that scares me is honestly the speeds of the two bays. I was under the impression the main HDD bay ran on SATA III while the optical bay was limited to II.
Let me know if you have any insight to this as I have a 13" 2012 MBP that I would love to do this with (along with 16 GB of RAM).