Stephen Lawler showed some really nice demonstrations of Microsoft Virtual Earth. They definitely have the "slickest" UI of all the web maps.
They've also baked in a lot of functionality that Google has left to mashupers (if that's a noun) and developers. The "Collections" for building sets of spots and locations is a very Platial and Wayfaring.
What is *really* cool to see is their Streetside, which allows someone to virtually drive through a city and view the forward and side-views outside a car.
Microsoft has a lot of muscle and they're really flexing it.