I have been watching the series for the first time this weekend, and I have to say...I love it. I'm very much an Arthur nerd. Meaning I know more about Arthurian legend and characters than most laymen. Though I'm far from fluent enough to be considered an Arthurian scholar.
I was avoiding this show because someone described it to me as "Hercules-style Camelot." It's not. It so much better. I avoided watching Hercules because someone's ego quickly took it over, and yes, the show was about him, so there was no way to divorce him from the plots without killing the show. I could never stomach how different the show was from the myths the few times I tried to watch. It just wasn't something I could handle. So I didn't watch. And I avoided this show for the same reason, fearing that it would annoy me in the same way.
It doesn't. There is one thing about Arthurian mythos that is different from Hercules. There are so many contradictory tales and rewritings of the stories that a new version of them only adds to the mythos. Now, granted, it could be done badly, but having watched ten episodes, I think I can now safely say that they've got people who know what they're doing plotting the story out.
Though they have aged down Merlin, it fits almost better than the older sage giving Arthur decrees and undecipherable prophesies from on high. He still has secrets from his prince, but the fact that they are learning from each other and growing closer is excellently done.
I think all the characters have been carefully studied before writing out the series. While Arthur still has his father, the basis of what makes him Arthur is there. And through their friendship, Arthur and Merlin grow together. (with some absolutely wonderful subtext.)
There are some major changes of character roles (Gwenivere as a servant rather than a princess & Lancelot as a commoner as well), but the way things are laid out, it's easy to see how the threads will weave to make the stories we're all familiar with. I look forward to seeing how it all plays out.