Cool if you have nothing better to do
Most tour busses will make a stop at the Pali Lookout at some point of their tour, but I don't think it's worth going out of your way for.
Basically the Pali Lookout is a observation lookout that allows you to look down over Kaneohe and Kailua. It's worth seeing if you're on your way to or from the Windward side of the island, but it might not be worth making a dedicated trip to. Once you lookout over the railing, it takes a few minutes to soak in the view, but that's about all there is there. Don't bother walking down the trail to the end of the Old Pali Highway, the roadway just ends and you don't get any better view along the way.