I understand that he writes urban fantasy mostly - a bit like Tim Powers, perhaps? I have only read one of his books - Into The Green. This one is set entirely in a fantasy world, it's about a gypsy named Anhgarad who discovers that she has the Sight, and certain magic powers. Her tribe is devastated by the plague, and she is soon caught up in an epic conflict with dark forces that seek to wipe out those with the Sight.
I remember the book as having a pleasing mix of folksy whimsicality and a heartening realism. The quality of prose was excellent, the plot was quite original (despite my inept summary) and the story gripping. I've been intending to follow up on more of his work for a while, but his books aren't exactly crowding out the fantasy racks here. Definitely an author worth trying, I'd say.
One of the things I liked about the book were the musical scores at the end - De Lint is apparently also a musician, with a particular interest in playing and composing in the Celtic folk style.