It starts off pretty decent and then kind of goes off the rails. It does a good job showing us the nature of Sully and Nate's relationship but extremely dumb story elements distract from the good ones. There are several red herrings and a lot of the actions characters take make little to no sense. The combat is annoying and there are less options than in previous Uncharted titles.
Basically, I'd call it the weakest game in the series, which oddly enough has the most interesting villain. The fact that it's not good is probably underscored by how great and well-paced Uncharted 2 was. It's worth playing once, at least. For me, Naughty Dog always starts out decent with the first game, knocks it out of the park with the second, and kind of get mediocre for the third game. This was true in Crash Bandicoot, the Jak series, and it's true of Uncharted as well. It'll be interesting to see how Uncharted 4 plays out.
If you haven't played it, it's super cheap at pretty much any location that sells used games.