I was very interested to see a Hawaii thread on the disboards.  We are long time WDW and DL fans.  We took out first trip to Hawaii in December of 2003.  We were given a week at a timeshare on Oahu (Waikiki area) and we added a week on Maui.  Since then we have spent two weeks on the Big Island in December of 2004 and another two weeks on the Big Island in March/April 2006.  We have plans to return for two weeks to the Big Island December 2006 and July 2007 (with 10 other family members).
Can you tell we have been converted?  Although we still enjoy our WDW trips, we usually combine them with a cruise.  After going to Hawaii, we realize how much more a vacation it is when we relax.
Us: We are myself (46), DH41, DS12, DS9.  We live near Seattle, WA.  We don't like to spend money on luxuries (i.e. Swanky hotels and high-brow eateries).  We aren't sunworshippers.  We like to "do" and "see" things.  A little souvenier shopping is all we need.  We like to live like the "locals" to a certain extent.
The Islands:  We adored Maui.  The beaches (we were in South Kihei) were uncrowded.  We learned to snorkel and boogie board here.  We are now hooked on both these activities.  We have our own gear (snorkels, fins, masks, boards) that we haul to Hawaii when we travel there.  We biked down Haleakala.  We drove the "Road to Hana" all the way around.  We took a snorkel boat to Molokini. We had a great time the week we were there.  The weather was sublime.  We had a condo across the street from Kamaole III beach.  I think we paid about $250 per night and it included a car.  It was a two bedroom with a somewhat distant view of the water.  We did our grocery shopping and eating out (Pizza Hut, Tacos, etc.) in Kihei.  Loved the Old Lahaina Luau.
Oahu:  Our week on Oahu was affected by 4 days of torrential rain.  Waikiki was empty (not a soul on it) when we headed down one day.  Wind was fierce and rain stung our skin.  We learned on this trip that the weather isn't always balmy.  We still had fun.  We did the Arizona Memorial (water was so murky you couldn't see the ship below the surface).  We headed to the North Shore and saw the crazy surfers at the Bonzai pipeline.  Some newbies on their boogie boards went into the surf.  One wave and they were all sitting on their boards on the beach with us.  We had two glorious days at Hanauma bay for snorkeling.  We liked the International Marketplace in Waikiki for tacky shopping.  We loved Lanakai beach farther East from Hanauma bay for playing in the sand and swimming.  It was too murky for snorkeling when we went.  We loved the pineapple plantation and the maze.  Try the ice cream!  It's a Dole Whip!  We stayed in a studio aparment in a high-rise a few blocks from the beach.  It was fine, but too small for our family.
Big Island:  We've now stayed on this Island for 4 weeks with 4 weeks planned in the next year or so.  We love to snorkel at Kahalu'u and boogie board at White Sands beach or Hapuna beach.  We stay in a condo south of Kailua-Kona.  We head to the volcano to see flowing lava (we were lucky).  The Fair Wind boat to Captain Cook is awesome.  We've taken two of their tours now.  We also enjoyed the Body Glove boat.  We've done the submarine.  We took a helicopter to see the volcano (pricey!). We've done two different luauas...Royal Kona Resort was okay.  Kona Village was great though pricey.  We get our condos from the vrbo.com website.  We've been happy with both condos.  $175-200 per night.  On the water (no beach) with beautiful views of sunsets, weather, boats, and dolphins. Walking distance to Kahalu'u beach. 2-3 bedrooms.  The Big Island has a Costco and 
Walmart....we use Costco for their deli a lot.  The pizza and dogs are the cheapest meals in Hawaii.  We love to hang out at our condo and play tennis and watch the water when we aren't hiking or on the beach.  The high-speed internet at the condo is great for keeping in touch.  I still feel there are a myriad of things I still haven't done....more hiking and exploring.  We still haven't gotten to the Waipio valley or the botanical garden or almost everything on the Hilo side...Kayaking to the Capt. Cook Monument...and lots more.
Guide books: we love the "Revealed" books that have been mentioned here and also the 
www.konaweb.com site.
Weather:  this was our biggest surprise ... probably because we visit in the "winter".  All three of our trips were affected by the weather in significant ways.  We couldn't go to the beach for four days when we visited Oahu because of torrential rain.  Our subsequent one-day visit since was impacted by rain the whole day we spent there.  Four days of our first Big Island trip were limited because all the beaches were closed for dangerous waves.  Our past trip to the Big Island had only one day of rain all day long with dark clouds and isolated rain for the entire two week stay....on the Kona side.  Very unusual.  Oahu was miserable during this time.  Rain for days and days with flooding.  I believe the Waikiki beaches were closed for a week because of the sewage spill.  None of this weather ruined our stay or even a day of our stay.  Our experience with Oahu however has kept us away from that Island.
Flights:  We usually get airfare for $800 from Seattle to Kona.  Since we are always traveling at xmas or spring break.  I book as soon as flights are available and they almost always go up....never go down.  Experience has taught us to fly directly from Seattle to Honolulu or Kona.  We got caught on a leg through Las Vegas on our first trip...never again.  Don't ever miss a flight to Hawaii.  The planes to Hawaii are always jam-packed and it can be days before you can get on a flight standby especially around the holidays.  I also book my condo a year in advance since the good ones go fast around the holidays.
Finally:  we would love to go to Kauai one day....but it will probably only be for a couple days.  We love the BI too much...  We would consider moving to Hawaii but it is too isolated, too expensive, and schools could be better.  We both work from home and can live anywhere.  We would consider a timeshare, but they are more expensive than just renting a condo through vrbo when we visit.  PM me if you have any questions I can help with.  I'm not very good at keeping up with the disboards sometimes....