Quite good! Stirring stuff and, giving it some thought later, don't there were any glaring anachronisms, none that really stuck out for me anyway (That is, items and attitudes that could not have existed at the time the movie takes place). Naturally, the princess is a bit more rebellious than one would expect and Jack (The Giant Slayer) is treated quite well by his social betters, but all in all, it seems pretty true to the period.
As I was getting some coffee for the movie (the local theaters carried coffee for a bit, so they've told me I can carry a cup of coffee in), the salesperson at the convenience store commented on movies plundering material from the past. Yeah, Jack is an adaptation of the old Jack and the Beanstalk tale, but yes, plundering the past for story material is hardly new.
The Golem was a 1921 movie that was based on old Jewish legends. When I saw it, it immediately brought to mind The Hulk from Marvel Comics. Thor, of course also from Marvel Comics, derives directly from Norse legends. The DC Comics series Fables (Started up in 2003), very directly and explicitly takes characters and situations from past stories and adventures and the TV series Once Upon A Time appears to have derived directly from Fables.
So the only question for me is, does the latest appropriation of the past do a good job of re-telling the old story? I think Jack does indeed do the job quite well.