We went recently and used the Go Oahu card (you can read my TR in my signature). I bought the 2 day card off their website when they had a promotion - if you sign up for their e-mails, they e-mail discount offers all of the time. I think, if you are planning on going to those places already, it's worth it. Look at the attractions on their list and see if they are appealing, then add up the costs for admission and see if it is worth it. You can go to the attractions more than once, but not on the same day. So, if you are planning to go to Kualoa ranch, you can go on different days to do different tours/activities.
I really liked that everything was paid for ahead of time. I'm not sure if we saved very much overall with it, but since we're in Canada and our dollar keeps getting worse and worse, it was nice to have everything already taken care of, so we didn't come home to a HUGE visa bill! (It was still pretty big, but that was just from food and souvenirs.)