I was able to use the original HDD for Time Machine backups and put a SSD in the original HDD spot, which is a great setup. Of course, I have backups going to a Time Capsule too. The only thing with my model MBP (mid-2011) is that I only get 3 Gigabit speeds, which isn't a huge deal since I'm mainly using the extra space for backups. I really like that I don't have to hear the DVD drive winding up and making spinning noises when I boot up anymore.
I did this upgrade this week and was successful. I definitely don't recommend attempting this if you've never done a PC build, studied PC repair, or don't have steady hands. It helps a lot if you have a spare laptop playing the OWC video and playing/pausing it as you do each step. Also, I wore latex gloves to help prevent fingerprints from getting on the LCD.
A tip for migrating to the SSD: install a new OS instead of cloning. After I installed the SSD, I could not get into the Recovery due to some error, which using Recovery is an essential step to migrate OS X from the HDD to the SSD. A way around this issue is downloading Mavericks via the Mac App Store and do a new OS X install. From there, install OS X on your new SSD, then use the Migration Assistant to move everything to your new drive, which it does really well for you. Then, you can toggle Startup Disk in System Preferences (in case you aren't booting using the SSD).
Oh, and don't forget to manually enable TRIM on your new SSD.