I realize that Arthur is not a real person, and the myths and legends surrounding him are just that. I also know that they have been changed and expanded upon over the centuries.
So, maybe I'm being picky when the fact that Uther somehow waged war on an ancient wizard with the same name as Arthur's future son bothers me. Or that, for some reason, in this movie Uther's brother is named Vortigern. The same name as the king Uther defeated, along with his brother Aurelius, in a war for the crown. Also, maybe there is some ancient tome I never heard of where Arthur trained in London's most prestigious Kung Fu school under the tutelage of the wise and mysterious "Kung Fu George" also known as "Chinese George."
All of that aside... No matter how much cinematic de-aging you do, Charlie Hunnam does not look like he's a teenager. I suppose maybe they were going for the idea that Eric Bana was a very VERY old looking 12 when Arthur was born and that Vortigern somehow hasn't aged a day in 30 years.
The acting was actually pretty decent. It's a shame the script lacked Ritchie's usual substance. Scenes that were meant to be moving and heartfelt came off as cheap and corny. It didn't help that we were never given a chance to care. Character development and story were left out and replaced with CGI and explosions. I went in expecting something on the level of Excalibur, and instead felt like I was watching... well, a Michael Bay movie.
As for the CGI, it would have been fantastic 10-15 years ago, and had it been used to augment and enhance well done battle scenes (check out Game of Thrones' most recent dragon attack). Instead, stylized CGI battle scenes were the only substance to the movie. According to legend, Arthur was a brilliant tactician, he won by out smarting the enemy. Planning ahead and using small numbers in guerrilla tactics to seem like a larger force, until his fame built up an army large enough to win wars. Guy Ritchie started to do this, but the guerrilla warfare lasted 10 minutes, then Arthur pulled out his magic sword and blew up London (I'm exaggerating... mostly).
I know Guy Ritchie can make good movies. Movies that mix action with a well designed plot, great dialogue, and deep characters. This, this is not one of them.
Anyway, long story short, it's a pretty horrible movie. As a fan of Arthurian legend, though not a true scholar of it, this movie was almost painful to watch, even in the background. If you like Guy Ritchie, go watch The Man From U.N.C.L.E or Sherlock Holmes. Even better, go watch Snatch. If you like Arthurian legend, go watch Excalibur, The Mists of Avalon, or even The Sword in the Stone. Heck, the BBC show Merlin, or even the old NBC mini series of the same name, were better than this. Better yet, do something Guy Ritchie obviously never did. Read a book. Go pick up a copy of The Once and Future King, The Forever King, The Pendragon Cycle, The Camulod Chronicles, The Lost Years of Merlin, or even Merlin's Descendants. If you're a really big fan, go grab Le Morte D'Arthur.