I have an '06 Duramax. I will NEVER go back to gas after owning this truck. It is hands down the best vehicle I have ever owned. 170,000 miles and all I've ever done is service it and put diesel in. 16 - 17 mpg at 75 - 80 not towing. 12 - 14 towing my boat (twins and lots of wind resistance) and never below 65 no matter what the grade. More power towing than you could need.
My opinion about the long term maintenance is the opposite of Jerry (sharp shooter). My last truck had the 7.4 and by 120,000 the constant maintenance $$ was the reason I sold it.
Unless something catastrophic or unforeseen failure happens, I'll be keeping this truck for a long time to come.