Piesport is the name of a wine growing village in Germany. On steep hills along the Mosel, some of Germany's best Riesling is grown. "Piesporter Michelsberg", however, only indicates that the wine comes from grapes grown somewhere in the area. It is a designation no quality producer with a good vineyard there would use, so when you find it on a label you are most likely looking at a mass-produced wine that will probably not even contain grapes grown in Piesport itself.
I bought my Michelsberg for £3.99 from Sainsburys as part of a blind tasting experiment in cheap German wine. How did it fair?