Windows 10 is a free upgrade right now. That changes after today. As of tomorrow, it will cost you $179 for the standard version of Windows 10 and $299 for Windows 10 Pro. The absolute last minute to queue up your free upgrade is Saturday night at 7:59pm AEST.
My experience with Windows 10 since my spontaneously forced upgrade earlier this year has been solid. The biggest problem with Windows 10 isn't the operating system Microsoft built, it's the unnecessarily aggressive method of pushing upgrades on users giving the OS a bad name.
I wouldn't be shocked if Microsoft made Windows 10 a free upgrade again in the future -- moving everyone over is in their interest, and most people are down with getting things for free -- but that's just speculation for now.