I've had mine for over a year now and love it, mines an 54 plate with 55K on it.
In terms of things that have gone wrong in that year of ownership equates to just one of the plastic cogs on the stepper motors for the ventilation system broke so got the inherant W203 clicking noise. This was a dash out job to fix but I did it myself.
Mine is due a B service and its time for the Spark plugs to be replaced as well so I don't expect much change from around £500 for this to be done at the local Merc specialist.
Mine has had its gearbox oil/filter changed by Ollie in the past when the previous owner had it and would recommend this being done for piece of mind sake.
I've replaced a few things more out of being **** then anything and they are cheap cars really in terms of parts it only gets expensive if you are paying someone else to fit them!!
Front Discs from Mercedes were £58 each, Pads were about £80 for EBC Redstuff. Fitting was an absolute doddle.
New front wheel bearing was £38 for the kit from Mercedes.
Average MPG is around 18-19 MPG to work and back which is approx 12 mile round trip fast A road and town driving. On a run she will do 28-30mpg at a very steady pace with the cruise on. Blasting around Cornwall its more low 20's.
In terms of ownership I get asked a lot of quesitons about the car wherever you go because its different/subtle/sounds amazing etc.... The biggest shock for most people is how cheap they are as from experience the average joe thinks its worth 3 or 4 times what it is but I gues thats what the 3 pointed star and private plate do.
I wanted an E55 but at the time it would not fit in my garage in fact now I am probably glad I got this over an E because it is a simple car to work on and I love the rare factor about them.
I've bought the odd thing for it new 19" AMG wheels, carbon diffuser, etc... but I am most looking forward to purchasing coilovers for it and a remap potentially in the future but a new house purchase has delayed this...