Music is just over 550GB on a 750GB SSD, nearly all ALAC, and nearly all CD rips. Back up occasionally to whatever drive is cheapest, but I might throw more drives into the ol' Drobo soon and offload that stuff into its own backup share as well. I moved my system drive to SSD first, for obvious speed reasons, but realized that with the constant access, the reading while writing, etc., I was going through media HDDs like crazy, moving my music to SSD next made the most sense. All of my CDs are in storage boxes, and all of my downloads get burned to BD-Rs and stored with the CDs. Some download services like eClassical are less of a concern than others, I'm mostly worried about tracking all the downloads from labels (especially non-English labels like Hungaroton), and those which revoke your right to re-download after so much time or so many downloads.
I will say that merely moving music to SSD for sound quality is obvious hogwash, but if you're using internal/onboard sound... Well, all those spinning motors sure can introduce some noise.