There are a few shops in Massachusetts which are very good at what they do, and this is by no means a comprehensive list:
Accel Automotive - they work on just about anything - including antiques, Ferrari, Alfa Romeo ( they are located right next to where Alfa used to be in the 90s in Waltham, MA). Good people & honest. I have used them for years and if they have ever made a mistake, they own up to it and pay for it. What else can you ask for from your mechanic? We have a Lexus ES that will likely never see another shop again outside of Accel, and the following shop for tires & alignment.
Donovan Alignment - now a franchise of American Car Care Centers but again - very fair & honest, very talented when it comes to diagnosis. I've never been disappointed.
As good as these two shops are, for our Mercedes-Benz, I recently started using European Auto Solutions. Due to copyright issues they can't actually have Mercedes in the name, but they work solely on Mercedes-Benz vehicles.
The short history is that the owner, Tim Allen (no, not that one), a former Fidelity big-wig, had a passion for Benzes. A specialty Benz restoration shop that worked on all Benz model years and types, located nearby in Wayland, and named Hatch & Sons, was closing up shop (they would eventually re-open in Hudson). Tim was at the point of needing a new challenge in his life, so he left Fidelity and under a year later, the former Hatch & Sons service coordinator was working for him in a new shop in Waltham called European Auto Solutions.
I have used them for a few jobs now. The insurance nightmare posts (look in the archives - I'm sick of even linking to them at this point) speak to some major door issues after my driver's door needed replacement, and EAS brought me in from the cold: they replaced faulty wiring, connected the window regulator properly, aligned the door properly, replaced the contact switch, etc. All for a very reasonable rate, and just knowing my car was in the hands of people who had a passion for the brand and who never touched other cars, that gave me a level of comfort I hadn't had in my vehicle since the accident in January 2009.
I also needed rear coil springs done, and those were done without a hitch.
During my last visit, the owner, Tim, had to give me a ride to and from work. He was telling me about his passion for the W124 chassis, or the 300E (and 500E) of about the mid-80s to 1992 or so. Speaking with the service manager, Ed, is cool enough, but having 10-15 minutes of time alone with the owner of the shop and talking Benzes with him was very cool. He had no issue at all with giving me a ride to work after their loaner car wasn't back in time for me as promised, which showed me he's a down-to-Earth guy who values his customer base. Being the son of a business owner who's been located up the street from EAS for the past 35 years, this is very important to me when selecting shops to whom I will give my business.
If you're in MA and have an old or new Benz, give them a shot, you won't be let down!