I live in Japan... if the people here are any indication sushi is incredibly healthy

. I have to be careful not to eat 40 plates of Nigiri and I'm good to go. A lot of the time I'll have one or two plates of 2 nigiri and then eat salad and sashimi. MMMM this is making me want to go out for some right now! The fish itself is wonderful you just have to watch out for all the starch and carbs in the rice and the fat that may be lurking in the toppings.
ETA:
I keep seeing people bringing up the risk of parasites... I mentioned this to a Japanese friend of mine and she laughed at me. "Food poisoning" is pretty much unheard of here and they/we eat a LOT of raw fish. None of it has been prefrozen. I'm not sure if it's just that Japan is so much more careful about food handling or if the risk isn't as big as it appears. It was an interesting conversation... but parasites don't seem to be a concern here at all.