There seems to be a lack of diversity in the food of Kauai as there tends to be too many pricey pan-asian fusion restaurants and too many generally tasteless crappy restaurants with little in between. Some of note: Mermaids Cafe in Kapaa was reasonably priced, inventive, healthy, utilized local foods, and tasty. Service was an issue as with most of the places on the island.
There have been some chowhounds recommending Dali's Deli in Kohloa. This place was pretty bland. The menu was very limited and the service was extra bad. I would avoid Dali's Deli.
Kintaro's sushi in Kapaa was excellent. I went there because it was recommended by a friend. The menu is very extensive and the fish was very fresh. And... the service was not only by far the best on the island, but great by any standard. I am a miso soup critic and theirs was the right combination of bonita shavings and seasonings. They have a dish which is escargot cooked in a mushroom and cooked in sake. It was interesting, but not great. I suggest sticking with the classics there, which they do well.
I went to Roy's. I have been to, and liked, the one in Monterey, and since the ones in Hawaii are supposedly better, I made reservations. It was an up and down experience. The appetizers weren't that exciting. They were presented well, but didn't taste as pretty. The main dish (must get the butter fish)was great. It was easy to see why everyone recommends this dish. The salad was so-so. But... and I know this is the most important for some of you folks... the dessert was awesome. It was a chocolate souffle that oozed out melted chocolate when it was opened up. It had a raspberry coulee sauce and some local Lapperts icecream on the side. Delicious. The best part, it was free. There was a mistake with our salad and it came out as the last part of the meal (before the dessert) and so the manager did not hesitate to comp the salad and the souffle. The service was fine besides the small mishap. Those are my picks of note. I had lots of portagee sausage and spam too. I grew up on that kine of stuff (my grandpa was Hawaiian).