I bought two of the $15 upgrades. I haven't installed them on any systems, although I have an MSDN copy from work running on one of my spare machines.

With one of the start-menu replacements (I use ClassicShell, but any of them are pretty much equivalent; Start8 seems the prettiest for those who care), and avoiding the use of a Microsoft account to sign in, there's nothing particularly bothersome about it.

I might not even exercise "downgrade" rights if I buy a new system this summer (as seems likely when the Haswells come out -- I want a better GPU than my current system has, and it seems better to wait to see what they put in comparable 14" business laptops vs. the heavier 2013-generation options rather than upgrading now to a heavier, larger option.)