Given that the hype for 10 ranges somewhere between "Utter Miracle" and "Second Coming of Christ", I must say that I was very disappointed when I freshly installed it on a spare hard drive.
Back in November, the Windows Technical Preview was awesome! It was basically Windows 8.1, just with a full-fledged start menu (one that allowed people to mix and match Windows 8 and 7 features as they wished, rather than basically taking the start screen and making it just take up a corner), and the ability to run "apps" in a window. There were no big privacy concerns (besides the keylogger, which I assumed would just be used for testing), and I thought that the OS showed promise as it was.
The release version is, quite frankly, a major let-down. They ruined the good start menu that they had going by basically making it a miniature start screen rather than allowing user choice, push "Cortana", which combines all of the uselessness of a digital assistant with the godawful search capabilities of Bing, force updates in spite of their history of pushing OS-breaking updates in the past, and generally act like this is a real upgrade over 7 or even 8.
The test system I installed 10 on was no slouch: it rocks some reasonably fast spinning rust, 8GB of RAM, an i7 860, and a GT640. Compared to a fresh install of 7 on the exact same setup, 10 is SLOW. This is because it spends all of its time grinding away, installing the already massive number of updates rather than waiting for the system to start idling or shut down.
In short, I'm likely never going to install 10 on one of my systems. My migration plan is to wait for 8.1 to die off and then move entirely to Mint, some sort of BSD, or even Haiku.
Dual Katmai Pentium III (450 and 600MHz), 512ish MB RAM, 40 GB HDD, ATI Rage 128 | K6-2 400MHz / Pentium MMX 166, 80MB RAM, ~2GB Quantum Bigfoot, Awful integrated S3 graphics.