Just did the NCL cruise and rented from Thrifty on each island. On Maui, Hawaii and Kauai the service was excellent and we actually got the car we reserved (mid-size) on Kauai. On Maui, they tried to give us a 2-door Jeep so I paid extra for a convertible. On Hawaii, they gave us a Ford Crown Victoria gas guzzler.
Unfortunately, we rented from Thrifty on Oahu also. First, they don't run a regular shuttle service from the Waikiki site to the pier so you have to call to get picked up. After multiple calls and frustration, we finally connected with the shuttle at the pier, but by the time we got our car, it was two hours after my first call. Once they assigned us a car (a 4-door Jeep, which apparently is a mid-size), we discovered they don't refuel the cars on Waikiki. The agent told me the car had 3/8 of a tank (closer to 1/4) and to bring it back with 3/8 of a tank. We also were told the Waikiki shuttle doesn't go to the airport, only to the pier so there would be a $30 drop-off charge at the airport. We turned the car in early with 1/2 tank of gas (no credit given for extra gas) and took a private shuttle to the airport for $12 per person. Very disappointing experience overall and while I would rent from Thrifty on the other islands, never again on Waikiki.