It's supposed to work on only the Android and iOS platform to give you a "real" mobile experience (not on a desk/laptop) but I found that it works if you are browsing with Google Chrome. If you don't have Chrome, I suggest you download it because it's lightning fast and integrates Google apps nicely. It didn't work when I tried it on the Android OS but it worked fine on the iOS.
I must say that the WP7 interface is simplistic and visually appealing. If I didn't use so many Google-based apps, I think that I would enjoy and much more appreciate a WP7 device. The look and feel is pretty different from Android and iOS but it seems intuitive enough.
To test out the demo, use the browser on your Android device or iPhone/Pod to visit http://aka.ms/wpdemo. No download needed. It also works on Google Chrome from your computer.
Does anyone have a WP7 device and if not what do you think of the demo?