As I've mentioned on other threads, Oregon's multi-lane roundabouts are absolutely terrible. The one at US-101/OR-202 in Astoria has, over the long term, resulted in a 150% increase in crashes, without any reduction in accident severity, and even pro-roundabout traffic engineers I've talked to concur with the notion of that roundabout being terrible. Given the wacky mess that was there before the roundabout, screwing things up that bad is quite a feat. Of course, given that it was built by ODOT, I can't say I'm surprised, sadly.
Even worse is the one at MLK Pkwy, Pioneer Parkway, and Hayden Bridge Way in Springfield. It cost almost $10 million to build, and it had almost 200 accidents between 2009 and 2013, with volumes only running 15,000-20,000 ADT. The city's own transportation study showed it having an accident rate nearly 3 times that of the second-place intersection on a per MEV basis, but because it cost so much to build, they aren't rushing to fix it.
I think single-lane roundabouts do have a place--situations involving weird intersection geometry on mid-speed collectors and arterials (30-40mph range, and especially if people actually use their turn indicators)--but multi-lane roundabouts have had a very mixed record nationwide, and they're prohibitively expensive.