If you’re a user of a legit copy of Microsoft Windows 7 or 8, the upgrade is free. Although, I’d suggest waiting at least a month for the initial bugs and software incompatibilities to get worked out 1.
I’ve been a Windows user since 3.1, but these days I only use Windows for gaming. Even in my limited use, Windows 8 was a huge step back in usability from 7. I simply did not need and was confused by the touch screen interface Microsoft bolted onto the existing desktop UI. I can hardly imagine how confusing it must have been for the non-enthusiast crowd.
With Windows 10, Microsoft has fixed the worst excesses of 8. According to reviews from Ars Technica and The Verge, desktop users will get a desktop focused UI, and tablet users will get a touch UI 2.