I just upgraded using the free upgrade from Windows 8.1. So far so good. <crosses fingers>.
The advice to wait a while until going to 10 is pretty sound. Unless you really need features of a new OS, waiting can help you avoid some of the early glitches. I've always been an early-adopter-glutton-for-punishment type that enjoys having the latest OS as soon as possible.
Many of the differences are interface related...giving you the option to use the newer Windows 8 interface, while restoring the Windows 7 start menu. It's not just picking one or the other though. They are integrated very well together IMHO, allowing you to use the best from each as you determine that to be.
Of course there are under-the-hood changes also...you'll typically see faster boot times for instance, always a good thing, and general performance improvements. I think...pretty early to be sure, at least for me...that gamers and those who use high-level graphics and other such applications will like 10.
FYI, here are a couple of links comparing Windows 10 with Windows 7. I looked at quite a few. I thought these were the best...
Windows 7 vs Windows 10 comparison: What's the difference? - Review - PC Advisor
Here