DH & I did this the first summer of sailing for POA. We loved it (funny how time can point out the good stuff and blur the problems!) We also spent a few days on Oahu before and had a mix-up as to where to wait for the bus to take us and our luggage to the port. We finally got that straightened out, but didn't know that our luggage wasn't taken to the port, since the first bus couldn't locate us (short story: we were waiting at one entrance and the bus picked up at another - both were at the bottom of an escalator).
We enjoyed the excursions. We went on the chocolate tour, showing where they grow it. Unfortunately, that is the afternoon that the chocolate buffet was on the ship (!?)
I love NCL's free-style cruising with regards to dining. Maybe more ships do this now, but DH & I like to eat alone. We made reservations at some of their signature restaurants too (worth the extra service charge - but make these reservations ASAP, before getting on the ship.)
I'm sure I'll think of more. In fact I kept a journal, so I'll go dig that up & post more later.