I have never had it. What's it like?
It is a rich heavy brew with low carbonation, dark color and full flavor.
Barleywine is an ale, not a lager like doppelbock, but think of a doppelbock consistency fermented longer (from a 6% alcohol content to a 9-12%), add more hops to the flavor and that's what barleywine tastes like.
It can get very flat very quickly, it can be overhopped and the alcohol content can get ahead of the flavor to the point where there is an unpleasant aftertaste.
Anchor Steam makes a decent, accessible one called Old Foghorn which can be pretty good unless it has been gathering dust for months in a forgotten corner of your local beer distributor.