It was exactly what I expected. Nice, well-constructed, well-developed adventurous fun with ancient classical gods thrown it. Could've been better, but then, it could also have been much, much worse...
The characters are a bit infantile, but I guess that goes with the "young adult" theme. The main characters are fine, just the supporting ones, like Percy's mother and stepfather, and his nemesis in Camp Half-Blood, could use a bit more work. But, like I said, I suppose it goes with the age grade. Maybe they're supposed to be written the way Percy sees them, and you can't expect much emotional depth and understanding from a prepubescent boy.
Sure, there are a few stretches and over-reachings, but it's a fantasy book. It's fiction. You are supposed to accept some things without questioning them too much. I do wish it wasn't so easy for Percy to get over all the obstacles, though. It's almost like he doesn't miss a step, nothing he does can be wrong. Sure, son of a powerful god, it's in his genes and all that, but instinctively knowing how to wield a sword well enough to battle the god of war? He did have practice... About a month of it. Really?
The work of adapting the gods to fit our world seems solid. I don't know how it's going to turn out in the next books, but it works in this one. There are lots of common elements with the Harry Potter series, but, in my opinion, there are enough differences... And I hope the other books will veer more clearly from it.