I generally don't spend many hours in a museum, so from my experience I estimated this one to 2.5 hours, after which I booked 1WTC Observatory. Well, I was wrong.
The most interesting and touching part is crammed into a single space, which is worth those 2.5 hours alone. Sadly, I didn't know about that and we took our time in the rest of the museum first, so we had to rush through the last two parts to get into 1WTC in time :-/
But the reason I'm deducting one "star" is: The museum is vast, it wraps around those gigantic memorial pools, which is clever use of the crater they digged at Ground Zero. But despite that, a cluster of visitors is crowding around way too small plaques describing the item. There's no reason it can't be quadruple in size, with much larger font for easier readability. That's the case on many places.
The aforementioned section is overcrowded, while other parts of the museum are nearly deserted. But overall they did excellent job and I can't even fathom, how they figured out those beams from the point of impact.