Newer diesels warm up pretty rapidly. I have a 5.5 mile commute and haven't had any issues. I do tow a lot on the weekends and put on quite a few miles (about 32,000 so far), so it isn't just a commuter and that might help. I generally don't bother with a block heater because I park in a garage at night, but if I was outside overnight I would use one.
I get about 16-17 MPG in my mixed cycle city/suburban commute, which I wouldn't say is abysmal at all. I actually think it's pretty darn good and it's slightly better than my previous gas V6 SUV in the same commute cycle. A Duramax should be in the same ballpark mileage wise.