This film is amazing.
I served in the US Navy's submarine force, and Petersen's movie is the closest you can get to being in a sub without actually signing enlistment papers. This is one of the few war movies that is not relentlessly propagandistic. Petersen presents a story of humans--not stereotypes, not jingoistic misrepresentations of sailors. By the end of the film, you'll probably find yourself cheering for the Kriegsmarine crew, or at least hoping that they get home safely. As an American, raised after World War Two, I grew up with nothing but negative images and stereotypes of WWII Germany and Germans. Petersen and his cast do an amazing job of breaking through those representations and portraying a story of young, proud men who defy the odds and the elements and do what they perceive of as their duty, even though they find no glory--just despair.
I would recommend that you read Herbert Werner's memoir Iron Coffins in addition to watching this movie, especially if you are unfamiliar with the Battle of the Atlantic. Also, after the end of the movie, you should go outside, stand in the sunshine, and be glad that you are.