Here's one guy's experience with a VW diesel:
"I love diesel, but I'm getting a bit fed up with the complexity of the emissions equipment. My wife's Touareg TDI has a urea tank, urea pump(s), urea heater, urea injector(s), particulate filter, two separate cats, and countless sensors. They keep finding bugs in the software to manage all this stuff (2 recalls so far), and the dealers don't know how to diagnose any of the codes thrown by the emissions system (they just want to tear the whole thing apart and replace each step in the urea chain....in a separate visit for each component). The urea itself is cheap, but the system is so complex even the VW-trained technicians don't have any clue how it works. "
http://www.autoblog.com/2013/09/13/mazda6-diesel-delayed-until-april/#aol-comments
The post is a comment in an article that says Mazda is delaying the introduction of its 2.2L diesel for the Mazda6 sedan until next April. The reason for the delay: Problems meeting the emissions and mpg goals.