I've been using Windows 10 for a couple of days now, upgrading from Windows 8. I don't yet know if it's much of an improvement. I really don't use any Microsoft programs or services, so while the upgrades to these are nice, I'm not likely ot ever see them (Groove music is a nice improvement over Windows Media Player - why both are included is beyond me. Then again, Windows 10 has both the new Edge browser and Internet Explorer. Whatever). The performance is nice for sure, but I have a fast PC and honestly Windows 8 performed well, too.
The start menu is, in my opinion, only a minor improvement. Definitely better because it's not full screen, but you can barely customize the all programs menu, which is something I've been doing since Windows XP (You can still put things in to folders, but it's so cramped, and the alphabet display is just annoying). I'm kinda hoping Stardock updates their Start8 program with some windows 10 features and compatability. I used that to make Windows 8 a happier place. (EDIT: After posting this, I went looking to see if start8 has been updated. Looks like they're making a Start10 instead. Good enough).
The free upgrade does make it worth it in my opinion. I don't think I would feel that way if I had to pay. Windows 8 would have been fine enough for a while longer.
I don't really have a verdict yet.