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.
Slow computer got you down? Maybe a hardware upgrade should be in your cards, if so this guide to the Fundamentals of Upgrading a PC is an invaluable read. PCSTATS Tips