I've been trying it for a while on my nephew's and friends' computers. I dislike the iOS-look, it just looks amateurish all around, from the simplified red/yellow/green lights to the bright blue folders to the flat design. Just awful.
I have Yosemite on my MacPro at work. It's as far as I dare to update as I still need InDesign CS4 to function.
I pretty much detest anything after Snow Leopard. Apple changed the minimum Finder window size in Lion so I have to use an Applescript to make my Finder windows narrower. I have three screens and a lot of open folders so it's necessary to have the windows narrower than this limitation Apple put on the OS.
My main problem with Mavericks is that it's got a SMB2 bug. After 24 hours any InDesign file I open off our Windows 2012 server and try to save results in InDesign quitting out immediately. The only solution is to restart or to force SMB1 by using CIFS. That of course results in slower file saves and file transfers and means my folders don't automatically update when new files are dropped in.
Yosemite fixes that bug which I'm glad of, but I still hate the look of the OS.
However, I am using XtraFinder which has enabled me to have labels again (I use these to determine what type of file/folder something is) and I have black windows with gray type. I believe you can assign any color you want, but I may be wrong.
Without XtraFinder I'd be blind by this point. I agree with your assessment of Fisher Price/Playskool design here.