This is, I think, one of the better installments of the 500 Kingdoms series. Mercedes Lackey has clearly taken to heart some of the lessons from The Sleeping Princess: the overall tone of the book is much more tongue-in-cheek and playful, without the attempt to make everything deep and dramatic, and also relatively free of (as another reviewer phrased it) the preachiness that has marked some of the other books in this series. In addition, the romantic relationship between the protagonists is handled very well, without the feeling of "I need a romance, let me throw it in out of nowhere" that some of the earlier books suffered.
Bella is a mostly likeable protagonist, although she still has some of those requisite "more practical than thou" moments. (They're a staple of this series in particular, and they get a little tiresome after the third or fourth book.) One point that I did find somewhat disappointing was the way her character development was handled: unlike the standard heroines of the 500 Kingdoms, Bella starts out with an unexplicated but quite visible selfish streak. There was one point in the book where she thinks that the werewolf can't possibly feel as bad about her situation than she does. This is a man who has lived as a complete hermit for five years in terror of the thought that he might cause exactly this situation! Bella does grow up, by watching how her family pulls together to deal with her absence. I just kept waiting for a moment where she would acknowledge that perhaps she did not give Sebastian sufficient credit when she first met him.
Sebastian is a completely endearing character as well. Like Seigfried in The Sleeping Princess, he takes one of the classic MALE stereotypes of folklore - in his case, the absentminded, unworldly scholar - and both plays to his stereotype and goes beyond it. He is unworldly, but he is also a powerful and skilled sorcerer who clearly takes his responsibilities seriously. (Which is part of why Bella's failure to acknowledge that side of his character grated somewhat.)
Oddly enough, however, my favorite part of this story was probably the villain. Villains are often a major weakness in Lackey's books - either they come from nowhere in the very end of the book (as was the case in The Fairy Godmother), or they are so blatantly, utterly evil that they're hard to believe. That was not the case in this book. On the one hand, it was more or less obvious who the culprit was by the end of the fourth chapter, which was one of the major weaknesses of the book - with only three central characters, there wasn't even a red herring to divert suspicion temporarily. On the other hand, the villain actually showed that in addition to his nastiness, he was also a responsible person in his own way, aware of his own failings, and even well-intentioned in a short-sighted way. (I also can't help but think that Godmothers and their ilk are so used to thinking of things in terms of The Tradition that they forget basic psychology and profiling in looking for culprits!)
Final analysis: fans of Mercedes Lackey will definitely enjoy this story; it's a fast, entertaining read, endearing, and it patches some of the more common plotholes in Lackey's writing. Those new to her writing should probably read The Fairy Godmother and perhaps The Sleeping Princess first - but this would be a good third choice.