We got a single-lane roundabout near my neighborhood, which replaced a somewhat dangerous intersection. It opened in late July, and I ride through it probably 10-15 times a week. My observations...

Pros:
- Slower traffic (cars max out at about 20 MPH, most travel the same speed or slower than me)
- Easier to get through intersection on bike - no more dodgy merging/left turns
- Better traffic flow - less waiting
- Slow speeds means serious injuries are less likely

Cons:
- Drivers do not signal their intentions
- Some drivers do not know how to yield when entering (my mailman got hit, in his mail truck)
- Difficult for pedestrians to cross - pedestrians are less visible, and drivers do not understand crosswalks in the roundabout

Overall, I think it's an improvement, at least on two or four wheels. (I also enjoy the Strava segment that someone made.)