I think it has been said before but I think it is important to remember that bad experiences relate to one dealership and not dealerships generally. I am very satisfied with the service I get at the MB dealer I use, although I have had a very bad experience at a different dealership.
At the moment I have to use a dealer to maintain my warranty. In future I will be very wary of using an independent. What stevebfl says all rings true and seems very wise but, nonetheless, there is some sort of reassurance in using a dealer.
One point of note: a lot of disgruntled contributors to this thread have older cars which dealers will be less familiar with.
1999 SL 500
1993 320 TE
1986 500 SEC