Have been driving the GLK250 since the end of June '13 and have had no major complaints so far. Zero problems (mechanical or electrical) encountered and it's simply a solid vehicle overall.
Have been averaging 30-31 MPG after 5 fill-ups in 80/20 Hwy/City driving. That's from my own mileage calculations but frankly the on-board trip computer is not too far off (within 0.5-1MPG accuracy). I'm not exactly a lead-foot but I don't drive like a granny either (I'm 28 years old).
This thing just eats road imperfection for lunch - I live in the NE region so there is plenty of that around. Can't wait to test out the 4Matic performance in the snow in another month or so.
TORQUE - loads of it (369 ft.lbs) from 1800rpm and up. This makes the car feel much peppier than it actually is (only 200HP for a 4200lb car). This is my first diesel and I'm already addicted. Fuel economy is just awesome for a vehicle this size.
Transmission is very smooth - shifts are almost undetectable. (The 9-speed currently being developed should be even better when it debuts within the next year or two.)
Column shifter seemed very ancient at first but then I started enjoying the extra space made available because of that.
Brakes seem little soft, backseats could use a bit more room, rearview camera and push-button start should come standard, navigation/infotainment system is in dire need of an overhaul/upgrade, both visually AND functionally. Power liftgate and pano roof should be selectable a la carte instead of being bundled with an expensive package.
Wish it didn't come with run flats (or, in MB terms, Extended Mobility tires) but the ride is perfectly acceptable to me (I tend to prefer a stiffer ride). They are just expensive to replace down the road.