I went to high school in Hawaii, and visited my Sophomore summer of college (my dad was stationed at Hickam AFB). And trust me, Hawaii is better than any brochure you've ever seen about it. Going from Hawaii to Tallahassee for college was such a let down!!!
The major con about living there is how much everything costs. Unless you have the ability to get stationed on one of the military bases (provided, of course, that you're already in the military), you'll be paying an arm and leg for rent/mortgage. And if you think gas is expensive on the mainland, then forget about it. Ditto with milk.
Another con would probably have to be how far away you are from everyone. Depending on which island you decide to live, you get antsy not being able to go very far. The local's call it "island fever". On Oahu, you can drive from Waikiki to the North Shore in under an hour (opposite sides of the island). And it would take you roughly 4 - 5 hours to drive the circumference of the big island (Hawaii). It gets expensive if you want to visit the mainland more than once a year, so if you get homesick easily, I wouldn't advise moving there. If you have the chance, take a travel position and enjoy the islands while you're there. Then if you fall in-love with the place, you can think about making a long term commitment. Kauai would get my vote for a place to live long term. Not nearly as many tourists as the other islands, and there's a limit as to how tall any building can be (I think nothing over 6 stories) ... not such a concrete jungle feeling like Waikiki can be.
The pro's are far too many for me to list. Visit www.hawaii.com
for some great information. It's making me miss the island just writing about it! I plan on going back once I'm finished with school, too. Maybe a graduation gift to my husband and myself for making it.