I didn't go for one. My Merc is a poverty-spec C180K and I saw no reason to pay extra for an MB approved car. The engine is a very basic 1.8 4-cyl Petrol engine (albeit supercharged), and the drive train in general is well proven and rather simple in comparison to others (VW DSG). And there aren't that many 'toys' in the car either... no sunroof, Xenons, or heated seats, etc. I believe that most repairs will be well within my means, and I would have to be extremely unlucky to need a complete new engine or new gearbox. Even so, my financial risk is limited to the value of the car (i.e. I can always scrap it rather than fix it!) which is constantly decreasing.
Had I bought a complicated or expensive car, e.g. any of the AMGs, then I would have considered an Approved car and proper extended warranty an absolute must.