The north shore is kind of boring in the summer - the big waves come in the winter. That is the main attraction there.
We once just took a drive around Oahu - did the circle route, and there are some very nice views.
I see no reason not to take it. but you could probably rent a car for less. Just map out your path carefully before you set out or you could hit lots of traffic.
We had some people drive us from Honolulu around Diamondhead and we did the full circle in a few hours - it was a LOT of fun. Try to avoid the inland highways - they get very crowded.