Both. I either hate to love them or love to hate them.
The first roundabout in my area, built over 10 years ago, is awesome. But that's because it's in a place where a high-traffic road intersects a medium-traffic road, and there are hardly any pedestrians. What used to be a 20 minute wait at a lighted intersection was immediately reduced to less than a minute.
But then they went all stupid with it, and started putting those friggin' circles everywhere, including downtown. Roundabouts are not designed for bumper-to-bumper traffic, and they sure as hell weren't designed for pedidiots. Downtown roundabouts should all be torn up and fed feet first into the pits of hell from whence they came.
But without pedidiots, they're great.