Besides the Lord of the Rings, this was probably the first real fantasy trilogy that I read. I've gone back and read it many times since then, and I think it will always be one of my standards for excellent fantasy.
There's something about Mary Stewart's Merlin that is just captivating. He's flawed, enigmatic, strong, admirable, calculating, naive, and wonderfully human. I confess I've always been a little in love with him. Mary Stewart makes his growth from child to the wise old Merlin so believable. Somehow she manages to show the mystery of his life and powers through his own eyes
. I'm still not entirely sure how she did that, but it will never cease to amaze me.
Another thing that I absolutely loved about this trilogy was the refreshing historicity of it all. This isn't a fanciful rendering of a stars-n-moon caped Merlin the Magician, but a gritty depiction of early Britain, caught in the middle of the shift from paganism to Christianity. The scenes are all depicted so well that I could literally feel myself inside them. It's been years since I've reread these books, but I can still feel the warm stones under my feet from the Roman pipes under the floors.