I had a disappointing experience with some shellfish I had obtained from the Byerly's in Edina (suburb of Minneapolis).
The three dozen or so littleneck clams seemed extremely fresh and flavorful and ALL opened readily upon cooking--but, they were LOADED with sand--this, despite cautionary measures taken ahead of cooking: specifically, I soaked these babies for 10 or 12 hours prior to cooking in an icy brine to which some cornmeal had been added (to get the critters to expel). I have never had clams as sandy as these, and one can understand, I think, how sand can distract from an otherwise excellent batch of linguine.
Still more disappointing: Out of a dozen oysters procured from Byerly's, one was extremely foul-smelling, dead or moribund, entirely inedible, and two others had nothing in their shells but a little bit of mud.
I don't live in the Minneapolis area, so I don't have much avenue for complaint--but if others have similar experiences, I would prompt them to speak to Byerly's about more attention to the quality of their shellfish.