The W210s did indeed rust badly and the interiors weren't on par with previous generations, but in purely mechanical terms, they were still solid cars. The diesels (particularly E250 TD and E290 TD) could cover 700k+ kilometres without any interventions to the engine or the transmission. The W211 is an improvement to the W210 in almost every aspect, and they are still plentiful on the roads in Eastern Europe.
True, from experience, the E290 TD was mechanically solid. The electronics, less so unfortunately. Ours was plagued by intermittent errors and beeping, together with some parasitic battery drain we could not trace down despite our best efforts.
I didn't have the chance to own a W211, but from what I read and heard, it was indeed an improvement. Even in the looks department!
>(particularly E250 TD and E290 TD)
Not a coincidence, though - these two still use those legendary OM602/OM605 diesels of its predecessor series.