Is Percy Jackson really that good?

There's a very good reason Percy is so dislikable - it's so he can grow and changes as a character later on in the series. By the end of the series he's really grown into a non-whiny hero. I would also say that Riordan gets better at his style of narration as he goes along. Keep going and you'll probably start to like them better. At least read to the Lotus hotel - I think it's about the most ingenious myth incorporation in the series.