I enjoyed sushi quite a bit when I lived in America, but since moving to Japan I have learned what "good" sushi is. Most American restaurants only offer mediocre imitations of Japanese sushi.
Even in Japan the quality can vary greatly. The stuff you get from a convenience store is not going to be as good as what you find in a restaurant, and not all restaurants are equal either. In some restaurants the sushi is so fresh that some of it is literally still moving. It can be startling to see your sushi twitch when you add a little soy sauce to it...
The best sushi is probably uni (sea-urchin), which has an interesting flavor. Maguro (tuna) is common, but varies greatly in grade. It's easy to taste the difference in quality. The best tuna is called "otoro", and is generally quite expensive.
Tsukiji fish market in Tokyo is surrounded by little stall-type restaurants with 3 or 4 stools in front of a small counter, but the sushi at these little places is the best you will find in the world. Make sure you pay a visit to Tsukiji if and when you visit Japan.
Here is some information about sushi in Japan:
YouTube - The Japanese Tradition - Sushi