Our family is one that personally does not enjoy O'hana, however, the concern I have for you is that you said that you did not want to waste MK time. If that is truly important to you then I would definitely reconsider going to O'hana, as even though it is on the monorail, it will still take some time to get there (if everything is working well) and then you will most likely have to wait. We went on our arrival day and we were staying at the Contemporary, with reservations at 450 pm. It still took us a bit to get there and then we waited about 20 minutes. I think you will really have to rank what is most important to your family when making this decision.
As far as the negatives to the restaurant itself, well, we just did not find the food great, our service was average and frazzled, and it was crazy in there with coconut races, a ukulele player, and lots of people. But, all of that is stuff that did not work well for us, so I am reluctant to dissuade anyone from a restaurant based on our opinions. I also think that I built it up to be THE dining experience while at Disney, and it just wasn't. No one in my family wanted to go back this summer.