I'm considering buying it. I have the current 5D, but it is not weather-sealed, and does not have LiveView. Those are both features I'm interested in for some of the Landscape photography I want to do. Currently I find that I always have to cover up or pack it in when the weather starts getting interesting (misty rain), and I'm tired of squinting through the Viewfinder while I'm composing a shot with a camera mounted on a tri-pod. The LiveView is not really mission-critical, but it would be nice to have.
I am awaiting the 5D Mk. II which supposedly will be weather-sealed and have LiveView before I make a purchase decision. Also, I prefer small cameras, and the 5D has always been smaller than the monster 1D line.
I have a lot of L lenses, but more importantly I also have Leica R lenses, which are another step better than what Canon makes. A recent review of this particular camera praised it, but pointed out that the sensor is now so good that the L lenses are beginning to show their weakness, and that Leica R lenses are another order of lens-magnitude better. Of course, the R lenses don't lend themselves to every kind of shooting, because they don't have auto-focus... so sports (for instance) becomes more difficult.
Fyi, there are good adapters for Leica R lenses to Canon EOS, but none for Nikons.
Leica R Lens collection