Hey there....I have had a 2008 CRD since new.....I have a build thread on here for more info.
Real world with the CRD has been good. From a reliability standpoint it has been great. Fuel economy has been a little bit of a let down though.
If you are running empty you will see 22-24 on the hwy. Purely city will be in the high teens.
4x4, brick design, and a 5 speed tranny all hinder the potential of this engine.
For example...I have a 2010 ML350 Bluetec with the same V6 engine.....I see 25+ mpg on that engine. Better aero, different 4wd system and 7 speed.
But...from an offroad standpoint almost 400lb ft of torque is unbeatable. If you do get one try to find one with QD2 or QT2. Both 4x4 systems are great...both offer a dedicated 4lo....the QD2 has fr/rr electronic locking diffs. QT2 uses a brake system. QT allows you to add a rear locker.