I've always loved the books so I may sound a bit fanboyish, but I'll try.
What really makes them good is the strong plot's and interesting use of mythology. The dialog can seem a bit corny here and there, but usually it can add some good humor, and for the most part the characters are interesting.
Another plus is the ability to make villains sympathetic and the good use of foreshadowing, though I admit the final battle seemed rather lacking, it's easy to overlook. The 3rd book is personally my favorite and the 2nd book my least favorite.
Overall, it has it's flaws, but certainly worth a look.