Taken as a series, Skulduggery Pleasant works better than each book on its own. The first book in particular is very fun, with pretty strong characters, good humour and some hints at more engaging plot to come, but it doesn't quite pack the punch you might expect considering what its parts are- something's... missing.
Then the second one follows up on this with some nice character development (though Stephanie, the protagonist, flounders a little here), an improved plot and a less episodic feel. The third one gets it as close to right as you could hope for in a not altogether serious YA urban fantasy; there's a very interconnected feel to the whole thing, especially considering the ending, and the author's really hit his stride with the characters. There's more dramatic impact, as well, with the series' concept being treated as less of a joke.
The prose in the stories is perfectly serviceable, but at times you can really tell the man used to be a scriptwriter- the action scenes are basically lists, and we temporarily step 'out' of the characters' heads as well. Also, he really, REALLY likes the things. The dialogue is good if a little stagey and 'fake' at times.
There's a lot of hints at some potentially great worldbuilding if the current style continues over the next six books. Everything's been consistent so far, and the 'clues' for the backstory sound like something genuinely interesting.
Overall, they aren't perfect, but they're a lot of fun and they're getting better every time. As kids' books they aren't patronising to their reader, which is nice. Also, it's always great to see a book likely to interest boys with such a strong and non-stereotypical girl as the identification charater. Definitely worth a look, either for yourself or any kids you might want to get something for, if you're a fan of more lighthearted fantasy.
Be warned, though, for younger kids there may be a couple of slightly... creepy
things in it. Not really a bad thing, though, is it? Nightmares are good for the soul.