Morro Bay, CA, is a small town that replaced a freeway offramp/frontage road 4-way stop intersection with a roundabout. Transit time through the intersection improved because you no-longer had to stop and wait for your turn. It seems like a good model for low to moderate volume intersections. Flow is increased and speed is reduced.
Roundabouts are the New Big Thing here in Sweden, they're building them everywhere. On average it seems to be an improvement, and I have a local anecdote where a very busy intersection was rebuilt and improved vastly (while "everyone" said that it was a bad idea before it was completed).
