So far the series has maintained a steady three stars for me. As a Middle Grade fantasy, it's great for it's target audience (juvinille males), but for me personally (someone who pretty much exclusively reads MG and YA fantasy) it's okay, but nothing that excites me.
That being said, this 4th installment and penultamate book in the series is the strongest of the series thus far. The reason why? Well, FINALLY the idea that the Olypian Gods are jerks finally started to be explored and verbalized. It is refreshing for Percy to encounter a number of characters on the opposite side who really make a good case aganist Olympus. My favorite scene among these being between our hero and the lovely Calypso.
In general, the story (and characters) start to mature and the themes grow more interesting. I am looking forward to the last book, which I will start tomorrow. Hopefully Book 5 will continue on this thread and make for a satisfying and compelling end.
I wouldn't go so far as to give it 4 stars, but if half stars were allowed, I would give it 3.5 stars.