I have so far upgraded 10 x WinTel PCs from Windows 7 Pro to Windows 10 Pro.
All of these PCs had SSD system drives, 4-core CPUs, and at least 4GB of RAM.
Once I learned how to use the USB installer (created using the MS Media Creation Tool), upgrades were quick and easy, and all activated OK.
I'm very happy with Windows 10 (but hate Windows 8 for all the usual reasons).

The next step was to try out Windows 10 on a (very) low-spec PC - an old ASUS EeeBox with a single-core Intel Atom N270 1.60GHz CPU and 1GB of SSD2 RAM, running a cut-down version of Windows XP on a slow 80GB 2.5" HDD. (No optical disk).
Step 1: a fresh install of Windows 7 Pro 32-bit booting from a USB
To my surprise this worked well (but slowly), and all drivers were installed automatically at the initial install.
Activated with a legit key.
Step 2: upgraded to Windows 10 Pro 32-bit booting from a USB
This was painfully slow (2 hours plus), but worked first time without asking for a key, and updated and activated automatically.
The Win10 EeeBox is slow, but no slower than it was on XP - great for the kids to play with.