If you're planning to use the bus to sightsee, I recommend mail ordering a Holo card in advance.
Cash fare is $3 per ride.
You can also purchase a Holo card at 7/11 or at ubiquitous ABC stores. They are not available at the airport or on the bus.
You can load cash value, one day pass, or seven day pass online. If you load cash value and take multiple trips in one day, they cap the fare at the day pass value.
The airport bus runs every 30 minutes. Luggage is restricted to carry on size. I used it last week and saw several people with larger bags.
The airport bus is pretty crowded and makes a lot of stops, so if you use it you're trading convenience for cost savings.
There are various bus routes you can use to visit Waikiki, Sea Life Park, Pearl Harbor, Diamond Head, Hanauma Bay, Turtle Bay. You can even circumnavigate the entire island by bus, though it will take more than four hours.
Many locals use the busses for work and recreational transportation. Sometimes they're standing room only for portions of the trips.
One advantage of using the bus is that parking is expensive and in short supply in many areas. Some routes get stuck in traffic but that will also be true if you're driving.
Overall their bus system is a good way to access major tourist sights and get a feel for local lifestyle. Though you can't be in a hurry if you use it.