I use an independent shop; owner is an award winning MB certified mechanic, and a great guy to deal with (Eurotech, Mercedes Benz and Volvo Sales and Service Center). I started using him after I had a problem w/ the fan on my S; I called the local MB dealer and they quoted me $600 in parts plus labor; when I called Rick he said "Check this xxx - I'll bet that's all it is", and he was dead on. $158 in parts and 30 minutes of my time earned him a nice bottle of wine and my business.
Look around, ask other owners, etc., and you should be able to find a reputable shop.

Good Luck!
KevinL