If you have windows 7, I would think about forgetting about the upgrade, at least until the last minute. You have roughly a year to upgrade for free, and lots of things can go wrong in that year.
I once upgraded to a Service Pack 3 in XP and it completely locked up one of my PC's due to a conflict with my AMD processor. I was forced to run SP2 after reformatting, and they never addressed the issue.
Windows 10 is basically a windows 8 with a start button and a handful of new technologies that they hope will catch on. If you have 8 or 8.1 and hate it, like my wife, then 10 is probably for you. It will make it easier for those who skilled 8 to understand 10, due to the ability to use the desktop like old school windows, as well as their tiles, which I hate.
I personally will wait at least a month or two to see if any major issues are reported prior to downloading the upgrade for my wife's PC.
As for mine, I have 7 Pro, and probably will until something new comes out, like 11.
IMO, the reason it's free is similar to the Xbox 360. They ran out 8 early, and skipped a lot of things, and realized it. So 10 is sort of a peace offering to get people back on board, and it's a complete 8. Much like the 360 was an engineering failure that they later corrected, after raking in lots of cash.