I really like his 4th Down analysis:
http://www.advancednflstats.com/2009/09/4th-down-study-part-...
The tl;dr version can be found at:
http://www.advancednflstats.com/2010/05/4th-down-briefs.html
The conclusion is that teams should go for it on 4th down much more often than they currently do.
He also has a calculator where you can get the exact values: