Mavericks (OSX 10.9) was free, but made a lot of folks unhappy with the file read/write issues it had. Apple tried to fix them, but never did. So we recommended skipping over Mavericks unless you couldn't avoid it ('cause you bought a new Mac) or because you needed to use an app that required it.
Yosemite may be ready, but it's too soon to tell. Files seem tom open faster, and that's good! But even though files open more quickly, it seems to use a lot of memory, and so far we've seen some system crashes. Let's give it a month and see if it stabilizes, and see if we can give it a thumbs up.