Main downside of using a VM is that it still seems a bit flaky with the graphics support, at least for me. Also some of the new mouse gestures don't work so well - e.g., moving the mouse to the edge/corner of the screen is hard, when in a VM your mouse overshoots the window. But it lets you get a feel of what it's like.
Also note that the "Dev Preview" is old - since then we've had the Consumer Preview, the Release Preview, and the RTM. The last of these is the one you want. Or alternatively, in 2 days it'll be out to buy.
Note that until 31 Jan 2013, people can upgrade online for only $40, though I think it'll be $70 if you want a physical DVD.
It is supposed to be a $70 upgrade for Win7 users.