I had a '13 lease SLK 250cdi AMG Sport for two years and loved it. So much so I ordered another one which I've now had for 18 months.
The only changes to spec I made was to include heated seats and airscarf (well worth having!) and the Bekker satnav (not worth it!) The '15 plates SLK came with red seatbelts which I don't like but it wasn't a deal breaker at the time. SWMBO likes them...
I'm not a high mileage user as I have three other cars and the SLK is just a nice weather toy, but, neither gave/have given any cause for concern, at all.
The power is plentiful (for my needs) and the handling is very good, improved actually in the '15 SLK due to Pirelli tyres instead of Continental in the '13.
In June the '15 goes back and I'm toying with going for the SLC. but am not convinced of the facelift front....
Maybe the C Coupe would make a nice change!