I agree 100% with HiVolt; the MBP begs for a SSD as its boot and application drive. My new mid-2012 MBP felt slower than my old 2006 MBP because it came with the 5400 spinning drive. (I had done the SSD upgrade to my old MBP)
For the mid-'12, I bought a second drive caddy to replace the optical drive ($42 from NewModeUS: »www.newmodeus.com/shop/index.php ··· s_id=410
OWC is also a great choice and a bit cheaper.
»eshop.macsales.com/item/OWC/DDAM ··· AMBS0GB/
I have a measly 60GB OWC Mercury Electra Pro 3G SSD and it's fine for OS 10.8 and lots of apps.
If you're going to do this upgrade, it's a bit easier to do it when new, before you start putting applications and data in your home folder, but with a bit of rejiggering in the user accounts prefpane, it can be done after the fact too.
I already had an external optical drive, but you can get an enclosure for the removed Superdrive to keep using it. I use mine maybe a couple times a year, mostly to rip DVDs for my kids.