Windows 8 seemed like a ridiculous sacrifice of desktop usability in an attempt to create a desktop, tablet and mobile hybrid. I didn't even feel a need to give it a chance, because it obviously sucked. Windows 8.1 didn't seem like a sufficient improvement. When I installed Windows 10 preview builds on my laptop, they were very unstable and still a mess due to various features transitioning to the Modern (Metro) user interface.
Ubuntu in Windows 10 made me want to try Windows 10 again. This time, Windows 10 seems usable. I could still list ways in which Windows 7 is better, but most of those are not a big problem. The duplication of features between the classic and modern interface may seem ridiculous, but now the Modern UI is more complete and usable. Lack of decoration in the Modern UI seemed ridiculous in screenshots, but after using it I find it remarkably okay and unoffensive. Its only big problem is bad text rendering in some apps, without subpixel anti-aliasing.
Windows 10 doesn't seem like a big improvement over Windows 7 in terms of desktop usability. Microsoft probably understands this, and offers free upgrades because of it. Otherwise, not profiting from upgrades would be ridiculous. Windows 10 mainly exists as a change of direction, still trying to unify the desktop with tablet and mobile interfaces, and trying to move applications to the Windows Store for profit. The aim is to create future profit, via smartphones, tablets and the Windows Store. I don't know if that part will be successful and worthwhile. However, the change in direction for the Windows desktop is finally succeeding. The interface seems like an alpha version in some respects, but it is usable. Many people are choosing to upgrade, or tolerating unintended Windows 10 upgrades.
There definitely are technological upgrades "under the hood". Windows 10 performs well despite running more services. It has some security improvements. It is clear however that upgrading to Windows 10 isn't going to make your applications run significantly faster generally. What Microsoft says about security seems more like persuasion to upgrade than a good argument, with no evidence of Windows 7 being successfully attacked a lot more frequently.
The most alarming changes in Windows 10 are those which reduce privacy and freedom. When you run Windows 10 you send who knows what to Microsoft. However, in practice this does not really affect you. People willingly give up privacy because they don't see real consequences. The decrease in control, for example with updates, might actually be a good thing. Those who really know what they're doing are still free to do whatever, and those who don't will find it harder to cause themselves problems.
I can't really say that I recommend upgrading from Windows 7 to Windows 10. The best I can say is that it's okay to upgrade. It's a good idea to take advantage of the upgrade offer for future use if it's really ending in late July, but you might want to go back to Windows 7 for now.