Have to disagree on this one; most of the designo interiors are very nice and MB do give recommended & non recommended combinations for those who need guidance..
Well they do like David Hasselhoff's songs over there...
On a more serious note, I've seen some fantastic designo interiors and some horrible ones. The darker colours generally are OK, but the yellow/sky blue/bright red etc... are an aquired taste.
Someone posted an ebay link to a W202 estate a few months back that was in designo orange and had a full designo interior (leather + wood), somehow that combination worked for me, but its a brave choice whoever specced it when new.