I chopped my SLK320 in recently for a BMW 330d. I did it for multiple reasons. the one that made me start looking was the fuel issue, but I also realised that if the cost to change is moer than a few thousand pounds, then that buys a lot of petrol, so there has to be more to it than just fuel.
however for me, I do about 15k miles a year, so fuel is a concern, but also, the SLK was never a keeper. it's only got 2 seats, my fiancee didn't like me putting the top down, and I knew we'd need something more sensible next year. However what kinda sealed it for me was seeing what the depreciation of the SLK was.
I agreed the trade-in price with the dealer in late July but the actual deal happened in early august. in that week, a new copy of glasses had come out and the price of the SLK had gone down by £650, in just one month.
I know that depreciation like that can't last, but I wanted to be able to get *something* decent for my car so I could get a good replacement and hence the deal was done.
interestingly, I've been tracking the old dear on autotrader since I sold it, and it's gone down in price by over £1000 since it went on autotrader in early august.
would I consider a big petrol again? yes I would, as others have said, the sound and power delivery is great, but BMW do make a damn fine engine and the 3l inline 6 in my 330d is a very good powerplant. so far I'm happy
dave
--keep your car in a good state of tune--check the fuel system regularly for leaks-- stay on top of tyre inflation--forsake wide wheels/low profile tyres--make sure any aerodynamic aids such as undertrays are in place---no binding brakes----decrease winter warm up times by fitting a engine preheater-- keep your top speeds down/avoid hard acceleration 
---use the economy setting on the autogearbox--- avoid peak traffic times/ road works restriction to prevent long periods idling at standstill---- buy the cheapest good quality fuel possible.
