I recently made a deal with a friend that I would read Lloyd Alexander's Prydain Chronicles if she would read the Lord of the Rings (since we had respectively managed to miss these fine books in our childhoods). I think she got the better end of the deal.
I liked the Prydain books, really. But they reminded me too much of a video game (Odious says I may be somewhat biased in my view of video games because of the ones he plays--yes, yes, it's true, he plays video games A LOT) in the linear quality of the quests and the lack of causality in the world itself. Prydain is too small a world for me to be really interested in it, though I suspect as a child this would not have bothered me so much. Also, the coming-of-age ploy is a touchy one--if you don't get it quite right it really doesn't work at all. I couldn't stand Taran--he was just a paper doll. There wasn't anything real about him, and his speech was so different from anyone else's that the dialogues were hard to follow. I did enjoy Princess Eilonwy to a certain extent, though if she were a real person I'd throw things at her. Gwydion was a paper doll too, so really I have to say the best character was Gurgi, with Fflewddur Fflam a close second. It's unfortunate that so often the main character has to be the least interesting (this is particularly true, I think, in TV shows) and the sidekicks are the ones everybody likes. Odious and I talked about this once, but I can't remember what our conclusion was.
That said, they're good books. I should have read them as a child (the reason for missing them is unclear, since our public library owned them--I think I was under the impression that my mother didn't approve of them), but they were still enjoyable in this first meeting. And the Welsh mythology is, of course, great fun.