I've run the version of Unity that is present in the Ubuntu 11.04 Alpha 2 release and it is very buggy. Some will wonder if I ran into hardware / video card incompatibility issues... but recent builds of GNOME Shell (from a nightly build of pre-Alpha Fedora 15) work fine on the same hardware.
With that background in mind it answers the obvious question of... why did the presenter show Unity through screenshots rather than a live demonstration of it? Either he was using a public computer (rather than one he owned) and didn't have it installed... or in its current form it was too buggy. Of course there are other possibilities but I think those two have the highest odds.
Ubuntu 11.04 has a little less than two months left until it is released... and the Alpha 3 is supposed to come out tomorrow... so I'll check that to see how it has improved. I certainly wish them well with the project, because if it goes well other distros will probably offer it as an option.
I must say that I do enjoy both GNOME 3 and KDE 4.