I ride one - sort of. A home-built Coroplast shell on my diamond frame bike (not a recumbent). It costs me about $100-150 to construct. I've been doing that for about 4-5 years. My apologies for posting in the Recumbent forum, but there's no such thing as an upright Velo so I'm taking some liberty to add my 2 cents.

There are some inherent drawbacks which make it an unlikely for general use in my opinion. Briefly, it's hot, less maneuverable, more trouble to store, somewhat heavier, and presents greater surface to cross winds. And as Robert said, can be ugly. However for specific purposes the advantages are greater than most people imagine.