There are a few folks on another MB list that I belong to that swear by the Chevron Techron Fuel System Cleaner($6-$8) dollars a bottle.
Then again, the head MB tech on that site says there's no need for chemical cleaners. He recommends quality fuel AND taking the car out periodically for a "spirited" drive.
I've experimented with Techron and the Lubro-Moly products and cannot tell if they've helped or not. I have noticed that by using Chevron Techron Premium(93 octane rating in south Texas), I get about 1.5 miles per gallon better mileage than with any other name-brand fuel. My 2 cents.