How does it taste?
Wasabi is a plant of the Brassicaceae family, which also includes cabbages. This plant is indigenous to Japan, and they can harvest all the year. The taste is so unique and very difficult to describe. Many Japanese people say it's kind of hot like red peppers, but it is not the best description. When you taste red peppers, the "hotness" comes on your tongue, but the taste of wasabi comes in your nose. Some English dictionary describes this as "freezing", and it's pretty near to the real sensation. Have you experienced frostbite? Severe frostbite feels like burning, doesn't it? It's something like that. By the way, in the movie, WASABI, Hubert "eats" wasabi, saying, "this is so good", but it is hardly realistic.
Wasabi and sushi
It's well known that wasabi is used to make sushi. However, the taste of wasabi is so unique that it can be kind of "hurting" if he/she is not used to it, let alone children, while many Japanese "adult" people enjoy the sensation in their nose. Therefore, some people order their sushi without wasabi to eat at ease. On the other hand, gourmet people never eat sushi without it; they believe that the real sushi cannot exist without wasabi. So, being able to enjoy wasabi indeed makes your Japanese friend believe that you really know how to eat real sushi.
Wasabi itself is neither salty nor sweet. So, you can't add wasabi just to your ordinary meals. However, the combination of wasabi and soy source is so brilliant that you never fail, and this is the easiest usage of wasabi. But don't use it too much, or it can make your meals "too cool"!