Windows 10 is the best Windows you can run today. This alone should make it a big win, not to mention the fact that it's "free." However I don't think Windows 10 is there yet. Can Windows 10 bring seamless updates that one or two years from now will make it the best, most polished desktop OS available, and still be called Windows 10? Microsoft must prove itself under this new direction.
Microsoft is set to release Windows 10 on July 29th and users of current versions of the operating system will get an upgrade free of charge. So where's the confusion? Microsoft has sent mixed signals (more than once) and speculation has built on top of that about who gets the free upgrade and who doesn't, if Insiders get a free pass or not. Here's an overview of the different upgrade paths to Windows 10 that explains it all.