Absolutely awful. I've waited 27 years for someone to bring this to the screen and all I have is a sense of crushing disappointment.
Right from the start, characters were not doing what the book had set out. The child Mordred was not Uther's son. Norwenna was not Uther's wife.
Merlin is a Druid of British Celtic origin. Hywel (Welsh name) is Merlin's Steward on the Tor. Lunete is Derfel's first female "companion" , is of Irish origin, and is not related to Hywel in any way. Morgan is disfigured by fire but appears whole on screen. When Gundleus attacks the Tor, Hywel is killed. Derfel rescues Nimue and makes his escape, picking up Hywel's sword. The sword is renamed "Hywelsbane" and features throughout the rest of the tale. That didn't happen on screen so will leave gaping holes in later episodes.
I lasted to half way through the third episode and then gave up. This ranks as one of the worst book to screen adaptations, only surpassed by "The Running Man"