While I agree that it is underrated, I have never not finished before closing time. Using FP efficiently makes it possible to be fiished by 2pm at the latest.
A good plan would be to head right to Kilimanjaro Safaris upon arrival. If the line is more than 20 minutes, get a FP and grab breakfast...or head over to Pangani Forest Trail...or if there is a show Flights of Wonder (you may have time for all three before KS). Then catch the train to Planet Watch. I rarely spend too much time there since there isn't much to do there (for me, anyway). Then go over to Kali River Rapids...it's usually a walk-on in the mornings. Afterwards I usually do the Maharajah Jungle Trek to dry off. Since I travel alone, I use the single rider line for Everest. But even then, you can grab a FP and catch Finding Nemo...or just get in the standby line. I don't really like Primeval Whirl (or as I call it, deathtrap spin), but even if you stand in line for it and Dinosaur, you'd still have time to see Lion King and Bugs Life before 1:30pm or 2:00pm. That's walking at a normal pace too (no running around). Then you can check out the animals in the oasis abd by the Tree of Life. It seems like a busy, full day, but if planned right you could do it all by lunch. But, that's me when travelling alone...I'm sure it might be a little more difficult when there are more people in the group. But it is still possible.
But I agree: I could spend all day there if I wanted and never be bored.
