Just returned from a week trip to the islands. I used this board a great deal in identifying potential restaurants and thought I would share my experience and observations:
Mama's Fish House - Paia - this is on many people's lists. Overall the food was very good and competitive with many of the other restaurants we visited. The atmosphere was great. We sat on the patio which was less crowded and provider a better view than interior tables. My wive had the Mahi and I had the Uku (grey? snapper)which is prapared the same as the Opakapaka (pink snapper) on their website. It is somewhat expensive but the four dinners at relly nice resturants all came within ten dollars of each other.
Moana Bakery and Restaurant - Paia - this was part of our Haleakala bike ride. Pretty straight forward - omelettes, breakfast burritos, pancakes, waffles, etc. Good and filling.
Paia Fish Market - mentioned on these boards and by some locals we talked to. The fish is very fresh and good. You won't be overwhelmed with experimental taste combinations or the restuarant interior. You'll order at a counter and sit on benches but enjoy very fresh and flavorful fish. I recommend the Mahi burger.
Pacific'O - Lahaina - we did dessert here a couple of nights. It has a very hip atmosphere and a nice patio right off the ocean. Of the three desserts we tried the the Lumpia - pineapple and banana in a warm springroll wrapping - was the best. It's served with a drizzle of chocolate sauce and banana ice cream.
David Paul's - Lahaina - a little bolder taste combinations than Mama's. My wife had their signature Tequila Shrimp and Firecracker Rice. I had the special which was a Opakapaka coated in sweet basil pesto and sauteed, served on mashed potatoes with a red pepper sauce. A lot of flavors but they worked well together.
Hawaii - we were in Hawaii for only one night on the Hilo side. The whole purpose was to hike the Volcano Park. I couldn't find many recommendations for restaurants on the Hilo side and the ride out of the airport was not encouraging in the places we did see. I was pleasantly surprised with the Kiawe Cafe in Volcano which is just outside the park. After several hours of hikes a simple pizza from a brick oven with a couple of Fire Rock Ales hit the spot. There are other items on the menu like ribeye, ahi and some pastas. They cater to the visitors leaving the park and are only open 5:30-9:30 PM. It's small and had a short line by 8:00 but tables turned quickly.
Nick's FishMarket - Waikiki - Pointed in this direction by a couple of local people I spoke with in Waikiki. We arrived and the Maitre D' noted that we were two minutes early. A couple of minutes later he tried to seat us at a table just off the bar. We declined and were shown to a more appropriate table within a couple of more minutes. This is a more traditional sea food establishment - not necessarily driven by what is the freshest local foods. Still the food was quite good and not a bad wine list.
Dukes - Waikiki - just a quick lunch here before climibing Diamond Head. I had the Ahi burger which was good. Again a good place to have lunch overlooking the beach just a few feet away.
Alan Wong's - Waikiki - as most point out the building is non-descript. The dining room was loud. My perception may have been clouded by the loud party standing at the bar (and right behind us) for forty-five minutes before being seated. We sat at the counter to watch the action in the kitchen. I had the sampling menu with the associaed wines. Really a nice combination of flavors. The flavor of the tomato soup in the soup and sandwich appetizer was fantatstic. I generally do not tomato soup but would have this often. The only entree that I thought missed was the poached lobster. The corn overwhelmed the delicate flavor of the lobster, but it was easy enough to push to the side. This was the best overall flavor experience. In pretty much any city the atmosphere would be considered below average. When you have to compete with open air dining steps from the ocean it is a significant detraction.
I hope this helps in some way.