I got a year of it through the humble productivity bundle (or something similar) a couple of years ago. It's one of the few things in the bundle that I continued to subscribe to (along with lastpass) after the bundle ran out. It's cheap, and I enjoy the service. I don't really use the premium features so much that their absolutely indispensable, but they are nice. I keep a huge library of stuff in pocket and the unlimited storage space and easier searching is nicer than having to re-google for things.
I never saw any value to buying a premium. Their free features did the trick, and the only reason I may even consider purchasing a premium for are the ads, which will probably disappear after Mozilla takes it over.