Lots of subjectivity to anyone's response, as well as models and MYs entering into the equation... Some being more problematic than others. While I have an SLK on-order and it will be my first MBZ, from the extensive reading I've done on multiple forums, I don't believe it will be anywhere as reliable as my Toyota/Lexus trucks and cars that I've owned since 1995, and fully expect if I tried to keep my SLK for an extended period, the cost to maintain it would be prohibitive.
A point of reference that got me thinking during my recent SLK financial negotiation is this... MBZ offers a maximum 7-year ELW. Lexus offers 8-years if you want it for that long. While price of the vehicles are about the same between a 2013 SLK250 and my 2013 Lexus 450h (hybrid) SUV, the cost of the MBZ 7/75 ELW is 30% higher than a more comprehensive policy from Lexus on the 450h for 8 years / 75K miles. I know there is apples-to-oranges in that comparision, but it's at least real numbers I know as data points. I mentioned this to the Finance Mgr at the MBZ dealership and received only a smile, then a story about a guy that bought a used Porsche from him and purchased an aftermarket extended warranty for $9K that paid for itself within a month. It seems to me even MBZ Financial knows there is going to be costs and as time/mileage increase, chances of a claim escalate to the point they won't offer an 8-year ELW, and if they would, it's cost would be far higher than something similar from Lexus which also does not have as many exclusions in the policy... Lexus has been at the top of their game with reliability and customer satisfaction surveys for a number of years in different polls. Again, that's subjective. You can shoot holes in every one of the surveys. You can see in my signature line the various Lexus I've purchased new -- all have been stellar in their reliability, be that my SUVs or the 8-cyl SC430 roadster I owned which was essentially flawless in it's performance over the nearly 8 years I owned it, with exception of replacing 2 struts to hold the trunk open around year 6 if I remember right.
So, before I get completely shot by MBZ enthusiasts in that highly subjective set of comments (and don't take a risk to respond again like this), let me reiterate that I have a 2014 SLK250 on-order. It will be my 2nd car (keeping my Lexus as the daily driver), and first MBZ that I'm looking forward to taking delivery of in August. If I love Lexus so much and believe they are so reliable, why am I purchasing a MBZ? Answer: Style, the ride and performance, the fact Lexus has no current generation vert that interests me, and my hope that MBZ reliability is better than what I had with my BMW 335i. ...but yes, I'm buying an ELW with my SLK, because I do believe I'll have problems after the standard warranty expires in the 5-7 years I'll likely own it, and if I do, I know it will be expensive to repair -- just as it is for any luxury brand (including Lexus when they do break outside of warranty). Hopefully I'll be pleasantly surprised, have no claims against my ELW, and have wasted my money.