I read these three books a long time ago and have to say they are really good.
I have re read them since and enjoyed them that time too.
They take a fairly truthful (I think) look at Merlin's life and do a good job of rendering it on to paper, telling of his exploits without getting too tangled up in all the magical side that honestly only Merlin could really tell you about.
10 out of 10 for me.
Incidentally Mary Stewart did another book called Thornyhold which is pretty good too.
It has a slightly mystical theme but it is part of the story rather than the backbone of it, which makes it more realistic I think.
Zygo