there's a few things to watch for:
Problem of premature wear of timing chain sprocket for balancer shaft of M272 and M273 V6 petrol engines. Mercedes Star bulletin about this. Big job. Can cost £4,000.
Watch for brown stains appearing on alloy wheels of C-Class at 2 - 3 years old.
Paint problem after 4 years on front and rear bumpers of solid red painted examples.
C350 model has propensity to blow 477 H7 dipped beam bulbs.
Strange problem of indicators flashing the wrong side. Might be due to moisture ingress to one of the side repeaters.
Uresolved dashboard rattle irritating some owners. Apparently MB is aware and is developing a kit to fix it?
Plastic interior door handles of W204 and S204 C Class seem to be breaking regularly. The first to fail is the drivers because it is the most used. Because these are an integral part of the door panel and not available separately, the cost to replace is £374 (inc VAT), plus fitting. Up to 75% is offered in 'Goodwill' in Germany. Nothing so far in the UK.
Glycol contamination of automatic transmission fluid via the transmission heat exchanger can still sometimes occur, necessitatiing a new radiator and heat exchanger and flushing out and changing of ATF.
13-10-2010: MB is recalling around 85,000 Mercedes Benz C- and E-class models in the US to fix a problem with power steering systems, the US traffic authority NHTSA said. MB told Agence France Presse that the recall is likely to be extended to other countries. The NHTSA added that a problem with the steering system's pump could lead to a loss of fluid leading to a loss of power assistance when parking. Daimler stressed that drivers retained control of the car even in the event of a power steering failure. MB said it will fix the problem in 2010 and 2011 models this month by re-tightening a faulty fitting.
07-01-2011: Diesel ML, C, E, S and R class: Fuel may leak. VIN from WDC1641211A626833 to WDD2183232A000277
26-01-2012: Diesel C Class , CLS Class, E Class, S Class, R Class, G Class, GLK Class and GL class: Fuel may leak. VIN from WDD2210261A374759 to WDD2210262A395344
26-01-2012: Diesel C Class , CLS Class, E Class, S Class, R Class, G Class, GLK Class and GL class: Fuel may leak. VIN from WDD2040011A512447 to WDD2043032F646730
1-3-2012. Alerted to Urgent Safety Recall for Mercedes Benz diesels. This relates to a possible diesel fuel leak which may develop at the fuel filter assembly. The dealer will replace the fuel filter.
21-1-2014: Heard of TSB to replace the Piezo electric injectors of 2.1 litre 250CDIs if customers complain of engine malfunction light coming on. Goes back to at least 2010 W204 C-Class.
30-4-2014: Just-Auto reports that Mercedes-Benz USA, is recalling 252,867 model year 2008-2011 C300, C300 4Matic, C350, and C63 AMGs built from 26 January 2007, to 13 July 2011. A poor electrical ground connection may result in the dimming or failure of the tail lights. MB USA is notifying owners. Dealers will replace the bulb holders if not previously updated and replace any corroded connectors, free of charge. Parts are not currently available. Owners will be sent an interim notification in June. A second letter will be mailed when parts are available, currently expected to be in August or September. Owners may contact Mercedes.