It's fun and it's free, what more could you want?
Hundreds and hundreds of mustards arranged by country, flavor, etc. There is a tasting bar, too, so if you want to try any of the mustards--ANY of them, no limit--you can do so before making a purchase.
There is also a pretty neat museum in the basement, detailing the history of mustard and showcasing all sorts of vintage mustard accoutrement. Very cool for history buffs and mustard lovers alike.