Your Wiki link is Wack! Probably because the magic roundabout is actually in Swindon not Hemel Hempstead.
http://en.wikipedia.org/wiki/Magic_Roundabout_(Swindon)
Roundabouts/Traffic circles can work well but in some cases were the traffic flow has become too heavy or too one sided, traffic lights become the only way to control them. Also as some body else pointed out they are not pedestrian friendly as the through flow of traffic can be pretty much constant.
A quick demo of how not to use a roundabout!
http://www.youtube.com/watch?v=LyyFi4puQ7s