I have been running Windows 8 on at least 4 computers since January when it was on sale. Every single application I ran in 7 runs exactly the same in 8. I don't use the Metro screen much, but I even find value in the live tiles when I do.
The OS has been stable, secure and fast. I have had no crashes, intrusions or infections despite connecting to public network all the time when I travel.
As best as I can tell, the people complaining about 8 are generally not computer-savy, and therefore cannot adapt to change when it happens. Even though the change here is nominal at best, it is still change.
BTW, I upgraded all 4 of my Windows 8 machines from various versions of Vista and 7. None of them were brand-new computers, and none came with 8 originally. All of them run faster, more stable and more secure after the upgrade. There is plenty of reason to upgrade a Windows 7 machine to 8.