Read them two years ago when I was teaching in a school and the librarian recommended it. The series is a favourite of hers. The first book, as Teresa says, is much more juvenile than the rest of the series and is also very different in tone from the rest of the books.
It's almost as if the first was meant to stand alone and was not intended to be part of a series. I've always wondered if she got the idea to write a series only after the first book had been published.
I enjoyed the next four books much more, especially because of all the myth she wove into the tales. For example, many of the adventures take place during the twelve days of Christmas, or at Halloween. The six signs take their power from the elements of which they are made: wood, bronze, iron, fire, water, and stone. The grail, the golden harp, and Pendragon's crystal sword reminded me of other older epics. I guess they strike a chord because we've heard them before and in our heart we remember.
And then there are the strange creatures from folklore which affected me most of all. Herne the Hunter (he's been an aternal source of fascination), with his stag's horns and his Hell Hounds; the Greenwitch, woven by night of fresh-cut green branches and then cast into the sea at sunrise; the Grey King, with his breath of cold mist on the hillsides.
I liked the books for the wealth of folklore and legend woven into them. They are in a way like Lovecraft's tales and the folklore give all these stories a feeling of being 'true' somehow.