We just returned from a two week cruise to Hawaii in May - it was just after whale season so we didn't see any, but we did see dolphins playing as we were sailing. From California it took us 4 days to get to Hawaii - it was wonderful and relaxing to have many sea days - we love to cruise so this was an added plus for us - 8 sea days total.
A cruise to Hawaii gives you a good "taste" of Hawaii, we now know where we want to go back to for a land vacation. Also, we sailed RCCL Radiance of the Sea - and like previous poster said, we were an older crowd - upwards of 50. The reason is the cruises are usually about 2 weeks long and cruises to Hawaii are usually in May and September/October - kids are in school - which is another reason why we choose the itineary.
Would I go again - in a heartbeat - as we love sea days!