I put 10 on my laptop (from 7) that I use for note taking and doing Max/MSP work for classes. It has been pretty good -- the fan runs cooler and the computer isn't any slower (can't say it's faster). I'm indifferent towards the new aesthetic and Cortana. It hasn't been a disappointment, though I have yet to see any major benefit to upgrading at the moment.
The start menu is better than 8, however I'm on the fence about it. Sometimes I like the 10 menu better than 7, and sometimes I say a few adult words and wish I was back in 7. Though, it might just be a familiarity thing for the time being, I can usually find what I'm after pretty quickly.
Cortana is useless right now, she doesn't do much; and I'm not feeling up to spending the time integrating my (Mac-based) phone and work computers with the Windows system (through Google, which seems the only solution). Hopefully they'll add some better functionality, but right now she is pretty much a voice-interface for Bing.
TL;DR: No complaints, no complements -- if they hadn't changed the UI I wouldn't have really noticed the difference between 10 and 7. Might as well upgrade while its free?