Usopp may be a liar, but his lying had started for good reasons– he wanted his mother to survive her sickness, and he wanted Kaya to smile and get better.
Plus, he uses lying to get ahead of (or away from) his enemies. Being completely human he has to use the run-then-attack method of fighting. He's got superb sharp-shooting skills, and he invented kabuto and climatact. He may not look like it, but he's also smart (fooled a couple of sky islanders to get his hands on some dials) and is some sort of a strategist as well. Ex I can think of for now is the fight with merry christmas and mr4 using their own tunnels; his psychological finisher on perona; using a smokescreen and detaching a car(?) to get away from the cp9 with robin with them.
Besides, his fights are usually creative, or funny. Or both. It's a break from the seriousness of the monster trio's (and the others') fights.
And let us not forget the awesomeness of this.
F3RL4OKOTWo
VOTE USOPP! We can do this!