The problem is most people don't adhere to #1 - "Cars entering a roundabout must yield to those already in the circle". Everything else breaks down after that...
I've been through a few in sleepy little towns in western PA, and they work well when there is little traffic. But then you hit one in NJ (where they are eliminating them) with lots of traffic and where the locals have no mercy...
That's another big problem with them - since they are fairly rare in the US, most people (including me) don't know how to navigate them.
Around here they are working well. Practice makes perfect I guess.