We spent 3 weeks in Hawaii and did the 7-night Pride of America cruise. We rented a car in each port and did what we wanted to do, when we wanted to do it, and where we wanted to do it for a whole lot less cheaper than what the cruise line charges for excursions. Plus, we went places they don't go and got to eat where we wanted to eat. It was so much nicer doing our own thing than paying way too much for shore excursions. In one port, we rented a care for the day for $25! And in another port (Hilo), they upgraded us to a brand new Mustang convertible!! And we only paid $35!
Definitely a much better way to go! Just have to make sure you shop around and pre-reserve cars as in some ports they'll sell out. At each port, they picked us up and dropped us off without any problems.
And one other note, in Kona, all the car rental agencies are at the airport, so it's expensive to take a taxi roundtrip. The only car rental company near the port is Thrifty.