Rode my 20 through some nasty winter weather and what is a stable bike in nice weather becomes a very nice bike in craptastic weather... friend here rode her 20 inch (non folder) through what was one of the worst winters we had and said it was one of the best winter bikes she has ever used.

Do prefer an mtb wheel in the winter as they do roll over and through deep ruts better and also allow for a wider and plusher tyre.