From a Windows Secrets email I just read:
"Who should get in line for the Win10 upgrade?
Again, the prudent answer is not to get in line at all. Wait a couple of months before considering the upgrade. I suspect that Win7 users will be happy to wait — that OS is familiar and solid. (Some Win7 users will probably wait for Windows 15.) Anyone who has struggled with Win8.1 will be tempted by the free upgrade; for many, the Win10 experience will be an improvement and possibly less disruptive from a patching perspective. I find the Win10 menu system much easier to use than Win8.1's native menu. And if you still want a classic Win7-type menu, you can always add Stardock's Start10 (site)."
"Obviously, reserving a copy of Win10 is entirely up to you. But there's no compelling reason to join Microsoft's marketing effort. However, if you do jump on the next Windows, I have one final word of advice: Make a full system-image backup before starting the upgrade process. It will probably be your only fallback solution."
Susan Bradley is a Small Business Server and Security MVP, a title awarded by Microsoft to independent experts who do not work for the company. She's also a partner in a California CPA firm.