MS reports that there are over 400m installations now of Windows 10, so one must assume it's not fundamentally flawed. There are 4 PCs in our household and all of these were updated to W10 from W7: two by update-in-place and the other two by a clean install. The laptop is 4 years old, two of the desktops are 6 years old and the other a year old. All the desktops were self-build and not bought with Windows pre-installed. I find W10 much better than W7.
The only issue I've had is with the graphics on my 6-year-old PC. It has the on-board Intel version which Intel kindly decided not to provide updated W10 drivers for. Although the PC worked OK, there were some formatting issues. All that was needed was a separate Nvidia graphics cost costing about £25 which comes with full W10 drivers and all is now OK.
A clean install of W10 often solves many problems with upgrading. I think it was a bold decision of MS to provide this upgrade path, and undoubtedly some people have had issues but I'm quite happy with the way mine upgraded.