The dealers - I've used two - have actually been pretty good. So far they have correctly diagnosed and repaired everything the first time. I've never had a take back for a problem, and never had a repeated problem.
My real complaint is more with Mercedes Benz North America - in my opinion they do a lousy job of supplying parts for this car. My first problem was a failure of the transmission electronics - it put the car into "limp home" mode, so I limped over to the dealer. They correctly diagnosed the problem the next morning - but it took 14 days to get the part, which had to be ordered from Germany, and shipped through the national parts headquarters in Baltimore to have the correct software version loaded.
About a year later the glow plug system went out. Again, the dealer diagnosed it in a day, but only had four glow plugs in stock. It required about a week to secure the remaining two. What I find odd about this one is the dealer was in Dallas (Plano, really) and the MBNA parts distribution center for the Southwest is in Dallas - out near the airport. We should have very fast access to parts around here!
Most recently air was being drawn into the fuel system. This time it was the dealer in Midland. Again they diagnosed the problem correctly. But it took them about a week to order and install the parts.
Other than a dead CD changer (swapped out same day) all my problems have been diesel specific. Perhaps this is an indication that I really should trade for an E430...