The replies of some above imply they are just upgrading their main Windows setup with the Win 10 TP.
My God, people, what are you thinking?
This is not a new release of Windows. It's a Technology Preview. It's not stable. It's not trustworthy. It's not even beta software. It's just a development build. And it's going to expire.
You want to test Win 10, great, more power to you - but treat the system you test it on as A COMPLETE THROWAWAY.
I would suggest testing it on a virtual machine or at most a physical computer that you had laying around and that had absolutely no use before - certainly not as an upgrade to your working system!
You DO know it's going to expire on April 15, 2015, and almost certainly not be upgradeable to the actual release, right? Many people were surprised that Windows preview versions expired (and they lost their programs and data) last time with the Win 8 previews.
-Noel