Well, it depends. The $39 fares you see are for flights originating or ending in HNL. If you want to take flights BETWEEN neighboring islands (like say Maui-Kauai) the fares are much higher. While I haven't yet tried to book it, a UA agent told me that I can take any two flights within Hawaii for the 5,000 mile award (like Maui-Kauai and Kauai-Kona). That would seem like pretty good value for 5,000.

Especially if you don't usually use the mileage plus program (I'm a Star Alliance gold thru US) and just accumulated some free mileage plus miles in one of the credit card promos!