I spent 3.5 years in the Bay Area (college) and I arrived in Oahu 2 weeks ago. I had never been to Hawaii before and I already had a job and housing situation figured out before I got here. I arrived with a suitcase and a duffel bag (travelling light

). Honestly it's not as different from the mainland as people would lead you to believe.
People drive just like in CA except we're going 60MPH instead of 90MPH. Driving in Honolulu/Wakiki is just like driving down Market St or downtown LA.
You still have your share of crazy people here.
The weather is a little different, instead of being cool with cool winds, it's warm-hot with stronger cool winds.
Housing is just as expensive as the Bay Area although it seems like places here are smaller than what you would get on the mainland. Housing prices here are still ridiculously high (judgement being that prices should reflect what the average salary is) but not as bad as SF. It's kind of interesting since my friend is relocating across the bay to SF and I'm out here and we've been comparing housing prices and living costs. I find it interesting that a lot of the houses here use window mounted ACs instead of central AC.
Costco is packed, out of the 3 times I've been there, it's always been full. I wasn't used to this since back in CA, I've never seen them packed beyond 30% capacity unless there was a special event where people would buy food to host/bring (superbowl, boxing, etc).
You'll pay about $1-$2 per item more on everything regardless of the store. Talking with some guys in the army, I've heard that as long as you have a CAC card, you can shop at the commisary since they don't check it closely. I may give it a try one day.