Like any vacation destination, food quality and prices run the gamut. Resort eats will be spendy; and some are downright outstanding. You'll find a number of familiar chains. Mom and pops are everywhere. There are food trucks and by-the-road pullouts; we love these. Etc.
A couple things stand out about island food: First, it's expensive to ship stuff there, whether good stuff or mundane. The upside is a wonderful slow-food movement/farm-to-table, be it tomatoes or cheese or island beef. Second, the islands are multicultural, and that richness is reflected in the food. Third, the seafood is almost unparalleled in freshness and diversity. I'll go days in the islands eating nothing but fish and shellfish and mollusks. Some would say there's a fourth -- Spam is omnipresent. lol
And we can't forget loco moco: rice, burger patty, brown gravy, fried egg.
Look for "plate lunches." Inexpensive combo plates that could be shoyu chicken or pork or katsu (fried stuff) or beef or fish, and the ubiquitous mac salad and rice, maybe a simple green salad. Ask locals where they eat. Try the trucks and roadside pullouts. There's a culinary style there called "Hawai'i Regional Cuisine," which is nothing more than a recognition of the ethnic diversity and cornucopia of fresh inputs available. In stylized form it's what you see at places by Roy Yamaguchi, Bev Gannon (I love every one of her eateries), Peter Merriman, Sam Choy and other of the progenitors.
Eat poke!!
That's one of the advantages of not staying in a hotel; having a kitchen. Some folks don't want to cook or prepare food on vacation, I know. But brecky for two not expended at some nominal hotel buffet spread, a dinner here and there, all locally bought and fresh, and you're talking the cost of a helicopter excursion, another night, whatever. Just another benefit to what I think is the better way to island stay.
Like Vegas buffets, I'm not exactly a luau fan. But you gotta try one. I think this is where the higher-end resorts are worth checking out (for the meal) instead of the review-like "extravaganzas" that draw in so many tourists; that is, unless you can hook up with a local and they invite you over for barbecue -- imu, the in-ground whole-hog (kalua pig) cooking tradition that is such a staple of family life and celebration.
When you settle on where let us know and we might have specific suggestions.
Invest in a couple guidebooks. You'll be money ahead. I'm a huge fan of the Wizard Publications guides: Wizard Guides And, no, I am not a Wizard writer. And I have "Frommer's Hawaii 2011" sitting on the table beside me as we speak. I see Wizard is now in smartphone app form. Cool.