I agree with Michele D about Dave of Passport Shuttle (also "Dirt Cheap Shuttle")--he can be a be a little rough to talk to, but the service is impeccable. And, he's nicer in person. Dave picked us up at HNL--right on time. Great communication--very easy to contact.
When going back to the airport from our Waikiki hotel, he sent Bill to come pick us up. Bill's really nice and his estimate of how long it'd take to get the airport was right on--I secretly timed him. =D Yay Bill!
Both trips were stress-free. Very simple. No confusions--and they give you a private minivan--which makes it even more comfortable. $30 flat rate + tip to/from Waikiki. Gets you to your destination quickly.