I've updated all but 2 of my computers, both home and work, to Windows 10. Of the two, I just haven't gotten to one. The other is an old Pentium D that I only use for one basic task and it's pretty slow as it it, so I might leave well enough alone with it. Most were running 7, but three were running 8. I also upgraded my wife's office computer from 8 to 10. So that's about 16 computers I've upgraded.
I only had an issue with one of them, and after doing a search, found that it was a reported occasional issue. Microsoft had a patch for the issue to fix it until a service pack came out. I noticed that they all did a major update recently, so I'm assuming it was the first service pack.
They are all running fine now. One that was an early dual core seemed to take a bit longer to start up than it did with 7, but otherwise ran fine. Another older computer I had upgraded took a dump, but it was a hardware failure that had nothing to do with Windows 10.
I like it! If I had to describe it...it has the operating ease of 7 but with the cools feature of 8. And even more cool features added with 10. The start menu is back, but the Metro screen is available too. Cortana is cool, but keep in mind that everything you do on it is tracked. Same with the new browser, Edge. It works very well, but MS records everything you do with it. I still use Firefox as my browser, and a non-Google search engine most of the time. It also has a cool new music player called Groove. It works quite well, and is a viable alternative to iTunes.
Certainly, faster processors work better with newer OS's. And I wouldn't recommend less than 4 gig of RAM.