Cortana, despite some shortcomings, is a very worthwhile addition, and the Edge browser gives Chrome a run for its money. Built-in apps are greatly improved.
Despite some bugs and annoyances (like being forced to accept Windows updates), it will be worthwhile to upgrade from Windows 8.1 before July 29, 2016, when Microsoft's one-year free upgrade offer runs out. Windows 7 users should consider upgrading as well, thanks to Cortana, Edge and the advent of useful Windows apps. (That said, you do have that full year to upgrade, and there are compelling reasons to wait a little while.)
As for me, I'm upgrading every Windows device I have to it.
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