Wasabi is hot. If you have it with sushi, you can control the heat by just adding a little wasabi to the soy sauce, and dipping the sushi in the sauce.
I have often had wasabi glazed fish dishes at restaurants, and it is often a milder heat, as the wasabi is mixed with other marinades.
Most of the wasabi that tends to be served in the average US restaurants tends to be a stronger, more concentrated version. I have had fresh grated wasabi twice (once at the F&W Sushi, Sake and Sashimi event, and once in NYC restaurant Megu). It seems to have less heat, is fresher, and it is recommended to add the wasabi directly to the fish, then dip in plain soy sauce. I learned that fresh wasabi is very expensive, and the use of it here in the US is fairly limited.