Just completed a 1300 mile trip with mine and got 25.6 mpg. This was 95% highway though at 70 mph. Around town I get around 22 unless like others have said, I get the itch to hear that exhaust, then it drops pretty quick. I bought mine a couple weeks ago and it is not nearly as bad as I thought it would be.
No input on maintenance yet as I have not had it in for service, but like most things, you get what you pat for.
I have an 06 SLK55 AMG. I have tracked it the past two summers on fuelly.com (2000 miles). My best result was 23 mpg (US) and the worst was 18 (a lot of city). I also have an 09 135 vert. I tracked that for one summer and never got over 22 mpg (US). Both have advantages and disadvantages.
On my 12 hour drive back from Illionis to South Carolina after picked up the car I got an average of 25.6mpg(US) at 70mph. I did make quite a few stops during the trip and also punched the gas a bit as well.
I am heavy footed and average 90mph on freeways.. can't help it :biglaugh: Have my fair share of tickets... Wifey already warned me with the 55 - says its a recipe for more tickets, but so far so good...