I am using it on two boxes so far and will be adding it to the third box:

1) dell desktop replacing vista (i reformatted the box and moved to 64-bit from Vista 32-bit)
2) dell mini replacing win7 RC (reformatted the box)
3) macbook via parallels (new installation)

You cannot upgrade directly from XP to Win7, you will need to do a migration. You can upgrade directly from Vista although it might be a good opportunity to clean up the machine. My Vista box is running better than it was with Vista. The mini is a little slow but that's to be expected with 1GB ram and an Atom processor.

All in all its been a good experience and its a good clean stable operating system so far.