Personally, I find the leather seats on the SLK 280 (and 350?) to be too rigid only because the leather in which they are covered is way too hard. It is as if the seat cushions were covered by 1/4" plywood. I softened mine by spraying on a liberal amount of 303 Aerospace Protectant, and allowing it to sit for a good 12 hours before buffing it off. (Note: there is probably a better product to soften "hard" leather, but I don't know of it.) This made a big difference, both in appearance and comfort. Indeed, I sprayed down my whole interior, leather and MB Tex, in this manner, because it was so "flat" that it looked and felt "dead" before this treatment.
If you don't think upholstery stiffness can impact comfort, consider this. Last Sept. I was one of those in the Mercedes booth at the major car show held at our governor's mansion grounds. I was the rep. from
MBCA. The then-new W204 C Class was featured. Some folks sat in a new W 204 that had the standard all-MB -Tex seats and said "the seats are too hard". I directed them to a W 204 that had the optional leather, which is quite soft. They loved it! Unfortunately, the leather in our SLKs is not the same as that used in the C Class -- it is much harder. More like the hard MB-Tex mentioned above.
If anybody knows of a better leather softening product than 303, please post about it -- as I'd like to know.