I've just started reading this. As Gollum accurately picked up from the reviews, it's a fairly predictable premise -- or rather two familiar premises combined. But so far it's done rather well, and Melling certainly has an excellent grasp of the myths and legends she is working with.
I'm sure the book has been updated for the reissue, since the two young protagonists make references to LOTR (the movies) which of course hadn't been made let alone released by then. I can see how these contemporary touches would make the book very accessible to the young teens it seems to be meant for, since it would help them identify with the characters of Gwen and Findabhair, and then suck them into the fantasy. (Just as Lorrie says it was for her.)
For the most part, the prose is nothing special, but every so often it sings.
(Although the verse, "Come away human child ..." is not, of course, Melling's own, and she does give proper credit to Yeats.)
There are a couple of scenes that I really got a kick out of -- like the way that Melling slyly lets it out that the old man is a leprechaun before Gwen catches on herself. (A car full of old shoes waiting to be mended.)