c43amg sums it up well, they are narrow seats with high sides. I'm about 75kg (160 pounds) and they fit me snugly. It is the first car I have owned where I am uncomfortable if I don't remove my wallet from my back pocket. My father who is slightly more solid than me found the seats uncomfortable.

Having said this, you don't slip around in them. The rear seats in the 190E Sportline would have to be one of the most comfortable I have ever sat in. They are also two well shaped buckets (fortunately, we have only two kids) and really do feel good, although I have not sat in them for a long period.

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107.023: 350SLC, 3-speed auto, icon gold, parchment MBtex (sold 2012 after 29 years ownership).
107.026: 500SLC, 4-speed auto, thistle green, green velour.
124.090: 300TE, 4-speed auto, arctic white, cream-beige MBtex.
201.028: 190E 2.3 Sportline, 5-speed manual, arctic white, blue leather.
201.028: 190E 2.3, 4-speed auto, blue-black, grey MBtex.
201.034: 190E 2.3-16, 5-speed manual, blue-black, black leather.