IMHO, the only Hawaii-Mainland product worth upgrading is CO IAH-HNL or EWR-HNL, because of the BF seats, and the food choices. The UA F product is not that good. Basically, you get a meal, free alcohol, and snacks based on the time of the flight.

I would try to buy-up to E+ plus in Economy if you are not an elite flyer with UA. The legroom is worth it.