I have completed uncharted 3 a few days ago and I will answer spoiler free.
first off I will say its definitely worth playing, what I say below is going to be a bit negative but I want anyone reading to understand my overall opinion of the game is quite high.
The problem I had with uncharted 3's length is the story. They had a really strong beginning and introduced some nice elements, but as the story went on nothing was really happening. The minute to minute story was great. However the overarching story was severely short and unfulfilling.
The gun play was ok and since I played it post patch was fine compared to uncharted 2, it was severly lacking in the hit detection. Add the fact the new melee fighting system left you open to enemies closing in on all sides draining your health and I feel in many ways the game was too long.
So over all the story was too short to be memorable and the gun play to weak to sustain a larger game.