Huh. I didn't realize it was free. I'm also still on Windows 7 64-bit, and assumed when the obnoxious little icon popped up in my tray with no way to get rid of it that it was a scheme to wear me down until I caved and bought it. At least, it claims to be free? I don't know. I'll ask around.
edit: according to their faq, it is indeed really really free
http://www.microsoft.com/en-us/windows/windows-10-faq?ocid=win10_auxapp_LearnMore_win10faqedit 2: update from a friend who is currently running the beta version - The only issue he ran into was one of the beta builds wasn't compatible with the processor on his Surface 3, but that had been in the release notes and was fixed. Otherwise it "runs butter smooth" and can revive some older/slower machines because they optimized it very well. He says that Microsoft is a very different company now than when it released Windows 7. He recommended reserving the upgrade, at the very least so you don't eventually get borked if you want to upgrade after they stop offering it for free. He has no idea what the strategy behind offering it for free is though. My guess would be that if they're actively trying to change their image it's easier to pull people in with a free upgrade, especially since Windows 10 comes with the new browser they're releasing to replace IE. Fastest way to make sure people at least try it out is to give it away for free to people who might not otherwise upgrade.