We should upgrade to new systems if they provide new functionalities that we will need. So I'm always interested in new stuff from apple (and their competitors). But if we don't really need the new functionalities, then we needn't bother upgrading. In the past I was very interested in mountain lion and mavericks because of airplay mirroring and both had a lot to offer with that. But I'm not so sure with yosemite though. It seems the main new thing is just turning your mac into a phone. Do you really need that though? I don't. The rest - changing fonts, the general look of things, etc, just seems to be about tinkering at the edges. And I criticise their youtube video. They say that "We reconsidered every element of the Mac interface, large and small. The result is something that feels fresh, but still inherently familiar. Completely new, yet completely Mac." Did they reconsider something central to my interest to mac os in recent years, namely airplay mirroring? Anything new there? Nope. Fast forward several months. Will 10.10.5 be better than 10.9.5? If not, I'll be sticking with 10.9.5!