I started using the Win 10 Insider Preview about 6 weeks ago. Auto-updated to RC a couple weeks back I think. Desktop system is about 5 years old, plus upgrades, SSD for OS drive, HDD for most of rest. Only program that didn't work for me post update was Acronis True Image 2012, which made me cloning my drives pointless. Lucikly was not needed. It took a bit to get used to it with double monitors, but I did my best to make the desktop win 7 - like for now.
I also installed Insider Preview on my Win 8 laptop that I had bought upgrade to 8.1 Pro for (clean install to get rid of that monstrosity). I didn't do this until after they insider preview downloads had stopped so was stuck with earlier build that I could not get to activate as Pro (which is only copy I had). Fresh installed 8 again with upgrade to 8.1 while awaiting for upgrade to 10 Pro.
Not sure exactly how it will work, but once upgrade to 10, supposed to be able to clean install to my hearts content. Having win 8 key embedded in BIOS would have been handy in past but pain in my ass now. I'm hoping they just upgrade the key to Win 10 Pro once activated to make life easy.