One of the suburb cities of my city has the highest number of roundabouts in the country. They started adopting them about 15 years ago, and have just kept putting them in. Other cities use them as a study of their efficacy.
The biggest problem with them is if 1 of the "through" roads is high traffic and the other isn't. If you're on the low traffic road you'll get stuck completely unable to enter the roundabout as non-stop traffic whizzes straight through with no gaps. It makes it far worse than a stop sign, and is essentially being stuck at a red light that never changes to green.