Any news on how to get a free 'reserved' copy? The Verge seems to suggest that you just have to wait, and it might take weeks, since Microsoft wants to do a gradual rollout...
It's probably quicker to download directly via The Windows 10 Download Tool: http://www.microsoft.com/en-us/software-download/windows10
Edit: can confirm crazysaem's experience. I did an in-place upgrade on a fully-patched 8.1 system and it worked flawlessly and relatively quickly (I kept my apps and settings). I'm not a heavy Windows user though.
Check out the FAQ: http://www.microsoft.com/en-us/windows/windows-10-faq