I see Usopp's role in the Straw Hat Crew as a low-budget Batman in the Justice League.
He's a normal guy part of a crew of superheroes, although Batman can hold his own while Usopp is largely comic-relief most of the time.
I don't understand the huge controversy behind his character on this forum. Those who say they are Usopp-fans want him to do something spectacularly outragous that saves the day when the crew seems likely to be wiped out.
I like the Usopp character BECAUSE he never has and never will do anything like that. And that is probably why anybody took a liking to him in the first place. He's a normal person in a crew of supermen, therefore the character appeals to me on another level than the rest of the crew. He DOES have some powers, but they pale in comparism to what his nakama can do.
I think he's the most complex of the characters on the crew. He's really weak and can't hold his own, and he knows it. His dream is to be a brave warrior of the sea, but he doubts he will ever manage that. He hides behind his lies to conceal his own insecurity, and has done that all his life. His pent-up frustration of being inadequate for his entire life finally resulted in him leaving the crew and obsessing over the ship. That's his biggest lie so far, he doesn't care more about the ship than his nakama. Proof: In Upper Yard, Shura attacked Merry and Chopper, resulting in the mast being destroyed. Usopp said something along the line of "don't worry, as long as you're okay" to Chopper.
I compare it to a child running away from home after quarreling with his parents. The child doesn't really want to leave, but he wants his parents to miss him and realise how much he means to them. I'm not saying that Usopp's a child, but think about it: he's in the middle of the Grand Line with nowhere else to go. Leaving the crew was his way of dealing with his emotional issues of feeling inadequate, fearing he may never complete his dream, angry at himself for not being able to protect the money, angry at the others for being so much stronger than him and angry because Going Merry couldn't be fixed.
It takes a brave man to accept an inconvenient truth. That's the kind of courage Usopp needs, not doing crazy stuff like sinking Buster Call ships or beating CP9. He needs to accept the truth that even though he will never be a superman like his nakama, even though he couldn't save Merry or the gold, even though he's useless in battle against powerful enemies, even though he will never face death or injury bravely, he's a vital part of the crew nevertheless. Then he will be a brave warrior of the sea.