Wife and I did on our honeymoon 3 years ago. Portion size was good, obviously the price was very high for meals bc it is hawaii. Who remembers what we ate, though? But the owner is very cordial and helpful. We ordered in advance, and when we walked into the hotel room the fridge was stocked with the food we ordered. If you are going to be going to any of the islands other than Oahu I would def call them and order well in advance; they will deliver to the other islands' hotels.
I was in Oahu in November and got food from Oahu Kosher several times in the week I was there. We did think about bringing food with us from NY, but really, when you think about the cost to buy it, and the pain it is to bring it on a plane and whatever prep work to eat it, it is actually pretty cost effective to get food from them. Plus at the end of a long day of touring or the beach it is nice to have a hot meal ready for you.
That said, all in all the food is pretty good. Portions are generous - if you are going with kids you can get one meal to split between 2 of them. We got nothing really fancy from them: grilled chicken and shinitzel, plus they have a separate Shabbos menu. Their challah rolls are really good, and it is the same food they serve at the chabad on Shabbos.
If you want to save some money and have a car, you can arrange to meet them to pick up the food in front of the hotel that houses the chabad.
Have fun, Hawaii is beautiful.