<---- See my Custom User Title over there? "Plays in traffic". Says it all really.
I live, work, shop, and ride recreationally in the city. I couldn't leave my apartment building on a bike without riding in traffic. Was it intimidating at first? You betcha. 25,000 miles later? Not so much. As it says on the back of the 50+ jersey, "Experience counts".
That said, I do use MUPs where I can, if it's during off-peak times. Two of my commuting routes incorporate portions of the MUP. But I commute at off-peak times.
I find the MUPs much more dangerous in peak times when there are people wobbling along, children, dogs, rollerbladers, skateboarders, iPod Zombies, and what have you. At least on the roads you know everyone is going by the same rule book. They may be ignoring them, but they all know what the rules are.