Love my '06 LBZ. Aside from maintenance costs, I've had to replace 3 glow plugs. The first my stealer did for $200! I then bought my own scan tool ($80)and a 12mm deep socket ($4) and replace them myself. Amazon sells AC Delco glow plugs for $19 each. It's like changing a spark plug.
I currently have nearly 90k on the ticker and get 11-13 mpg pulling my Pinnacle at 60-65 mph.
If I were in the market for a Duramax, I'd join the Diesel Place forum at http://www.dieselplace.com/forum/index.php
and poke around looking at some the problems the various models of Duramaxes are experiencing. However, don't let the negative posts scare you. All of the big three diesel manufacturers have similar forums that are used for problem-solving sites.