I drove my friend's brand-new SLK last weekend. It still has the german plates on it from when he did the European Delivery.
It's a NICE car. LOTS of torque, and for the money you get a heck of a lot more luxury than you do with the Boxster. However there's some questionable bits--like why is the windblocker still a mesh knit stocking that you put over the headrests? It makes a lot of strange glare from headlights behind you at night.
The thing I liked the most about it was the fact that there's even less wind turbulence in the cabin when the top's down than there is in the Boxster. Nice!
But it has a totally different attitude than the Boxster. As a "driver", my Boxster S makes me much happier than his torquey SLK. He's the kind of guy that asks me not to redline the car (yes, it's broken in). I'm the kind of guy that tells him to beat on it!
And personally, I like the way the Boxster looks much more than the SLK. That beak-nosed thing that Mercedes has been doing lately is rather gaudy I think, as is the pie-plate sized Mercedes emblem/grille.
Oh, and the steering is way too light on the SLK.