Your basic doomed-lovers story. He's an Army Ranger and she's a soft-touch socialite. They keep in touch with daily letters, the last of which is a Dear John letter. Strange, because since his name is John, they all are Dear John letters. Never mind that.
Amanda Seyfried is getting better with every role (Mean Girls, Veronica Mars, Mamma Mia) and Channing Tatum is perfectly cast as a GI (He was literally GI Joe last year). Dear John is a decent romance, but the ending seemed tacked-on, like the studio added it after the original ending scored poorly with test audiences. Either way, I liked it enough for one viewing.