So far so good.. Not too many complaints. Some interesting design choices (bad choices, poor design) especially with respect to context menus. But, overall, the system looks good. Feels a lot like a flat version of Vista rather than 8. Cortana is a cool addition as well. Dunno how often I'll use it though.
I've reported one critical error so far. It's reproducible by forcing a shutdown by installing any program that needs to restart the machine while having other applications actively consuming IO resources (like VirtualBox or VMware in my instance). This causes a catastrophic crash that was only fixed by shelling to a command-line as an Administrator and forcefully shutting the machine down.
A few of my associates and I are considering pen-testing Windows 10 to look for vulnerabilities. Selling exploits to large corporations or even the government is a very profitable business.
Anyway.. to anyone thinking of upgrading, I'd say hold off until some of the bugs are fixed. It's obvious that some updates are required. However, if you're just upgrading for DX12 and gaming, and don't use your PC for mission-critical work, then fuck it. Back up everything and go for it. It's definitely a more usable operating system than Windows 8.1.