I don't usually read Health Affairs, but this article was highlighted today so I read the free article:
http://content.healthaffairs.org/content/early/2012/07/09/hlthaff.2012.0327.full
Long story short: this particular ACO saw lower medical spending growth and quality of care went up.

aqc-chart.jpg
http://www.washingtonpost.com/blogs/ezra-klein/wp/2012/07/12/in-massachusetts-an-accountable-care-organization-thats-actually-working/