From what I've heard Win10 is ok. I had the notification a while ago and having done a smallish amount of research decided to accept the offer. I've not done too badly over the years skipping alternate versions. Win 7 has on the whole been good, 8 a pile a crap so 10 should be the all right version
I did have to laugh though that the first feature mentioned when trying to sell me the free upgrade was stating that the start menu was back....
Someone on a podcast I listen to has been using the preview version if Win10 for some time and says generally it's been fine, there have been certain updates which have borked it, but then that's kind of the point of the preview. Sounds like people on 8 will probably want to upgrade, people on 7 will notice less difference, which can be a good thing...