Windows 10 looks to be a minor improvement over 8.1 in most all ways, and a massive improvement over 8, and you can't really argue with free.
Look out though, a small number of AMD processors don't support the security features Windows 10 requires, so they can't do the upgrade at all. And if you have a VIA processor, well, I've no idea. Pretty much any Intel anything from the last several years should be fine.