I got interested in this movie because of the naval battle involved, The Battle of Myeongnyang (Wikipedia) in 1597. Wikipedia claimed of the movie itself that it was not only Korea's most financially successful movie, but also very well reviewed. Rotten Tomatoes (as of today, 2016-01-10) has four reviews, three of which are positive. The battle itself is fascinating: the Japanese had routed the Korean navy in the absence of the Korean's star general Yi Sun-sin - he was betrayed and tortured (historical fact). After their replacement general proved totally incompetent and got most of their ships destroyed, they put Yi Sun-sin (played in the movie by Choi Min-sik) back in charge. Except he had only 12 ships against a Japanese force in excess of 300 ships. Yi's choice of the battle location in a narrow straight with extremely strong currents proved to be even more advantageous than expected.
I was put off early by the painting of the Japanese as slimy and repugnant - they're even wearing heavy make-up to make them look more evil. The Koreans are, of course, good hard-working citizens. Most of them are terrified - who wouldn't be at those odds, they knew what they were facing. And the writers had the decency to include a treasonous captain who tried to leave (also historical fact). But given that the battle was 400 years ago - and that the Koreans and Japanese aren't in any kind of conflict at the moment - I'd kind of hoped their view of it would be slightly more neutral.
Some critics loved the drama of the large scale battles: unfortunately, I saw CG and took issue with the wake thrown up by speeding oar-powered boats. These are fairly large warships powered by men below decks with oars. Instead, they moved about as if powered by motors (odd, that).
Where I have to give some credit is on something else I thought was a bit absurd: the battle is initially taken up only by the Korean flagship, the Admiral's ship. I thought that was ridiculous ... but in fact it's what happened.
The movie is historically interesting, but if you're interested in the history you'd be better served by reading the Wikipedia article linked above or finding a book on the subject. As a movie drama it falls down by being too jingoistic and lacking in nuance.