From my experience, Dell laptops - one from 2013 and Alienware 2015 - both had no problems upgrading from Windows 8.1. So its an improvement - most issues I had with Win8.1 are solved in Win10.

On the desktop with Windows 7 and motherboard with overclocked CPU from 2009 (meaning stuck only with Win7 chipset drivers) - no lag, no bugs, performs well BUT I have BSOD, corrupt component store (sfc /scannow) - I think Win10 is more fussy with this kind of setup. I've disabled overclocking and seeing if it helps...

For those on Win7 with older motherboards with overclocked CPU, proceed with caution.