Spicy. I guess it's similar to wasabi (some cheaper brands of both are even basically the same thing made with horseradish). You don't need much, don't squirt it on like American mustard. But it goes really well with red meat, or on a ham sandwich.
Hm... I asked because I thought I hadn't had any, but now I wonder if that's what the kind of super hot mustard my dad always made from a powder was. It does have a sort of horseradish-y spice to it. Burns the nostrils like hell.