I've been playing with it here, and did a few test runs.
I did two installs of Windows 10.
First install is a fresh install.
Second install is an upgrade from my Windows 8.1 partition(Which I did back up just in case)
I didn't clock in what the boot time is but it's not much more than Windows 8.1 for the fresh install. The upgrade took longer, maybe 10 seconds longer.
Fresh install I used drivers for Windows 8.1. The Nvidia control panel wouldn't install, I've tried setting compatibility mode, run as admin, and even take control of it and still refused. So just took the drivers and installed them.
Since it was previously installed in my upgrade partition, the Nvidia Control Panel opened but did nothing. No values changed when I tried such as basic things like resolution or rotation.
The "Start Menu" feels very cramped. Does not feel right with a mouse. I know that this is just a very early preview of it and it is subject to change, but this does need to improve....
Open GL is broken. Doom 3 runs but there are weird textures popping in and out, and is very slow. I thought it was because of my mods such as Sikkmod and Wolfens textures, but even after a fresh install on both the upgrade and fresh installs did the same thing. Quake 4 runs, but defaults to the lower ARB rendering. Trying to force ARB2 rendering does nothing.
Later I'm going to do some scaling tests with it.