There is much less overt magic in Stewart's version of Merlin. He is brilliantly intelligent, a shrewd reader of men and psychology, educated and wise (though in these books he is not very old). He has 'power' -- mostly prophecy, but he can perform sorcery when necessary, but his powers are not as great as he lets people assume. In The Hollow Hills, the famed sword Caliburn is not plucked out from the stone as happens in the traditional legend, but is discovered by Arthur in a cave (where Merlin has hidden it) and there is a bit of showmanship, orchestrated by Merlin, to win over the doubting warriors. But Arthur has already proved himself on the battlefield, as well as being acknowledged by Uther, so there is less dependence on the magic or trickery to raise him to his throne.
However, a lot of this book is taken up with political jostling and ambushes in forests, neither of which I'm especially interested in, and I felt the story was dragging in the middle somewhat. But I'm looking forward to The Last Enchantment, and toying with the idea of reading some more Arthur versions this year. Does anyone have a favourite to recommend?