I've played with it and so far have ZERO reasons to switch...

Real benchmark tests, not fluffed up frills:

http://www.techspot.com/review/1042-windows-10-vs-windows-8-vs-windows-7/

Plus the Cortana issue; she is a massive data collection system that so far we have not found a way to uninstall her. You can disable her front-end but collection still goes on. Hence the 'free' upgrade. Right now I stay with 7.