I had bought my former 7-series used, and drove it way past its warranty period. It gave me some trouble initially when its water pump/radiator leaked and had to be replaced. Fortunately, that was a very known issue for 7-series, so BMW had drastically reduced the part price and the whole thing was fixed for under $500 - non-dealer (ironically I had to spend way more when a similar issue came up on my wife's Japanese car). Other than that, no significant electronic or mechanical issues up to 120k miles, at which point I sold it.
Personally, I think these cars are like a live time bomb, and I have now completely swore off used or out of warranty cars (at least premium brand cars). I have seen enough friends go through very expensive repairs on their Mercs and Bimmers, when they are out of warranty, so I don't think its a great idea, unless you are a big time DIY kinda guy who loves to "fix" things yourself.
Thats what I am afraid of. Right now there are pretty good deals out there for cars due to the rising fuel costs, but I am more concerned about the repairs. I would not be using the car for every day commuting but would like to to go on longer rides (Visit my parents ~ 200 miles/ summer home ~ 45 miles) so I need that 'confidence' to be on the open road. With my 325, I felt that I could be on the road on a whim (like a Friday evening).
I didn't know that Mercedes no longer offers the warranty, perhaps that is why they are less expensiv then BMWs? In terms value, Bimmers do hold up well, so might just go with the CPO route. In the US they are offering 2.9% financing which almost makes me wonder if I am better off financing and using my car fund for investment purposes with a higher return.