Early Wesley was pretty awful. Being given Gene's middle name, and the kid being a genius, it was obvious it was an ego thing for Gene. Once Gene took a backseat in the writing, Wesley became likable.
Season 3 onward, he became a much better character. A great start to season 3 showed that he was just as infallible as the next guy. He was good all the way up to his departure, and his occasional visit. He was great in The First Duty. Journey's End, though, what an ass.