I've now completed the five book Percy Jackson cycle from Rick Riordan (The Lightning Thief, Sea of Monsters, Titan's Curse, Battle of the Labyrinth, and The Last Olympian), and on the one hand, it's really obvious that Riordan comes from the popular fiction writing tradition (he also writes a series of mystery books for adults), but this is not really so bad a way for young readers to pass the time.
Popular fiction writers tend to be people who write in the very literal, description heavy fashion that people who don't look for a lot of complication but plenty of stimulation from their reading tend to enjoy, and that's why so many more people read this kind of thing than more literary fiction. On the surface, it's not very "good" writing, in that it's not terribly nuanced, but again, it's readable. Riordan, for instance, seems to have written as if he got most of his inspiration from the way cartoons are presented to kids, with very deliberate and not overly serious characterization (even from the villains) in most situations (though Last Olympian gets some major points by delivering on the dramatic potential of the prophecy at the center of the narrative finally coming to pass).
In addition, he adapts most of his storytelling directly from existing Greek myths, taking whole characters and situations with very few changes and transplanting them to modern times, sometimes with a twist and sometimes without really thinking it all the way through. One would have wished that he had gotten some editing tips from the start. There's no real reason to have dragged the story to five books, except that he could get his target audience to continue buying these books no matter what (which is why he's now begun a spin-off series and an unrelated one based on Egyptian mythology which he's writing at the same time).
Still, it's not really so bad, but its long-term potential is questionable, especially the less readers look for that next Harry Potter. That's the main hook for Percy Jackson, and the more distance from Harry's release comes between existing readers and those who might come next the more likely Percy will be forgotten...and Harry won't. Harry completely outclasses this competitor.
I wouldn't discourage anyone from reading these books, it's just, unless you're looking for a fun, easy read, I would not go out of my way to recommend them. I don't want to sound condescending, because I did read through all of them in less than two months, which is to say, I was willing to continue reading them even though it's not the level of writing I normally pursue.
If there are future editions of the series, I would strongly urge Riordan to be a little more transparent, and include notes on all the original myths he adapts in notes either at the beginning or end of each novel. Readers will either be existing scholars of Greek mythology, or should be motivated to become ones, and Riordan would do well to make it clear that almost everything he writes except the individual campers already exists, and in the exact form he presents it. It wouldn't be so bad if Percy weren't presented as an ignorant narrator who is always relating events after the fact, and never seems to be remotely interested in learning about anything he's just learned is basically in his family tree. It boils down to the fallacy of Riordan believing his audience is as simple as he cares to write.
But still, I will stress that they're not, after all, more of a chore than a pleasure to read.