Our coach is a 2011, and has a DEF system. Like Marty and Diane, we've had no trouble with the system at all. It's only a minor PITA to fill every few thousand miles, but even that's gotten easier with more DEF dispensers at truck stops.
When we were looking at coaches, I researched DEF systems, because I knew nothing about them. My first instinct was to buy a 2010 non-DEF equipped coach, because in my experience, pollution control devices usually caused mileage to suffer, and other engine related inefficiencies. I learned, however, that DEF systems were so effective at treating the exhaust, that manufacturers could re-tune their engines to be more efficient, and ultimately get slightly better mileage.
I wish I could do an apples to apples comparison with an identical non-DEF equipped coach, but that's obviously not practical. In any case, we've had no problems at all with our DEF system.
Kev