I've been playing with Windows 10 through the Insider program since it started.
I ran Windows 10 Technical Preview in a VirtualBox virtual machine. While Windows 10 always ran well, the installation and upgrade process wasn't smooth. I believe that was due principally to the virtual environment.
I have an old ThinkPad T61 that was still running XP. All I did with it was run iTunes offline. That seemed like a perfect opportunity to load Windows 10 on. It had 2GB of RAM and a 60GB hard drive. It was a Intel Core2 Duo T9300 64-bit.
I had a copy of Windows 10 Enterprise so why not?
I deleted the XP partition and redefined it and did a clean install of Windows 10.
It was an absolutely flawless install. Everything worked the first time.
There have been a couple of things I've fiddled with. The wireless adapter doesn't always reconnect after restarting. I haven't searched to see if there's a better driver for it. The TrackPad center button doesn't scroll on
I fought with the desktop theme. I prefer to use a desktop image with a white background. Windows 10 insisted on using a white font for the desktop icons with a black drop shadow. If I turned off the drop shadow, the icon text simply disappeared. In a minute I'll get to the solution.
With that under my belt I moved on to a Windows 7 instance on a ThinkPad X220.
The X220 was running Windows 7 Pro 64-bit and was eligible for a free upgrade. I followed these instructions and forced the upgrade in place.
From start to finish the procedure took just under 3 hours. Again it was an absolutely flawless install. Everything worked the first time.
I haven't had any problem with the wireless adapter and the TrackPad center button works like it should. I had already hacked around with the desktop icons to get them looking like I wanted them under Windows 7 and they stayed "fixed." The X220 had Office 2013 installed and after a brief "installation" dialog all the Office apps worked fine and retained all the settings. Even little things like power settings stayed intact. Oddly all the printers went away but were easy to reinstall. And Windows 10 has its own "Print to PDF" service. Microsoft Security Essentials quietly disappeared.
Back to the T61 desktop. I exported the desktop theme from the X220 and installed it on the T61. Then again turned off the drop shadows. After logging off and back on the desktop was fine. I've tried everything with the TrackPad driver. The driver dialog doesn't have the "ThinkPad" tab like on the X220. I even tried reinstalling the driver in Windows 8.1 compatibility mode. I used Device Manager to uninstall the device and let Windows reinstall it. No luck.
So far I've stuck with the new Windows 10 Start menu. I don't like it but I think it'll be Ok. It's taken both systems a while to successfully build the index and search right.
Overall, I couldn't be more pleased. My "Big Honker" server offered to install Windows 10 Pro today and I declined for the time being. That system has a couple of applications that, although cleared by the compatibility checker, I'm concerned about, e.g. Picasa, Tivoli Continuous Data Protection, and a USB scanner. That system is 5+ years old and I wish it had USB 3. It may be simpler to just replace it.