I suspect this board will soon be full of comments from over-emotional people praising "Dear John" as a "pearl" and a "rollercoaster ride" and all the other vacuous words this film's target audience typically employs.
I am most definitely not this film's target audience, but I do not dislike romantic dramas either, as long as they are well made, so here is my objective take on the flick.
It is not good.
It's not a bad movie either. But the plot meanders, development stagnates where it should've been moving forward (right around the middle, to be precise), and as for the ending...it almost felt as if they had run out of ideas so they suddenly said, "Hey, let's just film a last scene real quick, put some sentimental string soundtrack over it, and end it that way." Even Amanda Seyfried's beauty could not save this. Channing Tatum too gave a good performance, but you can only do so much with a flawed script.
Speaking of the music, it is unbearably predictably and kitchy. From the smokey voiced, irritatingly high-pitched female folk singer schtick (surely chosen to appeal to the majority of college-age girls that will go see this movie) to the overused "shimmering strings and piano" combo, it only annoys anyone paying more attention to the film as a whole rather than to his own "feelings." The film has a good beginning and the major conflict that launches us into the second act were all promising. So was part of the second act itself, as the story unfolded. Then the film just dropped the ball. Beyond that, I'd have to give spoilers.
"Dear John" is not a bad movie, but it doesn't work as it should either. If you want to see a truly moving film about prolonged love waiting to be reunited, go watch "Notebook," which was truly superb.