Are great for maintaining vehicle throughput, but aren't necessarily great for bicyclists and pedestrians. The video below, of a roundabout in the Netherlands shows how few vehicles stop or yield to bicyclists. Only one vehicle really does--a motorcyclist.
The interesting thing, and of course this is because it is in the Netherlands, the reality is that there were more bicyclists waiting to cross the lane than there were vehicles driving across the crosswalk. In other words, for throughput purposes, crossing--at least at this time of day--should have favored the bicycle lane because it carried more traffic.