The first time I tried it I wanted it plain just to see what it "really" tastes like. So I didn't add any mustard, daikon, pickles, sauce, anything. And I was pretty unimpressed. I am a big fan of stinky cheeses, so I was expecting something, erm, "robust" like roquefort or gorgonzola. But natto by itself really doesn't have all that much flavor.
But once I started adding stuff to it I loved it. The cool thing is finding new stuff to add. However once thing marks me as an intransitive gaijin -- I cannot and will not eat natto for breakfast. Everybody has their limit, and mine is breakfast.