I enjoy historicals and I enjoy time travel so this film had double interest for me.  Plus the cast drew me in - I love all these actors.
I found the opening scenes, the situation in the present, unnecessarily confusing, but it does set up the situation wanted - ie 2 groups (one NK & one SK) of soldiers sent 400+ years in the past, to the Joseon era, where they meet up with Admiral Yi Sun Sin before he entered the military.  Action scenes fighting against Chinese invaders, and heart-warming brotherly bonding occur, as the soldiers try to convince the young Yi Sun Sin that he should give up ginseng smuggling to become the great military strategist he's meant to be.  All this does not offer much innovation or new perspectives, but it's very well done and the plot had sufficient twists to make it interesting.
Kong Hyo Jin as the sole female in this film does add some interest overall - not love interest but perspective & comic relief.  I'm sure that including her in the story explains the scenario at the beginning.  She acts somewhat as a narrator, or at least a narrating presence - she observes rather than plays any active part but she's always interesting to watch.
All in all this did entertain me and even moved me.  I thoroughly enjoyed the film.