The storyline is absorbing and immersive, with believable and entertaining characters and dialogue. It's definately got plenty of replay value as well, due to the different classes and storyline options.
Though still essentially linear the story flows nicely and gives you enough control to make you feel like you are part of it, and not just progressing through an interactive movie.
It would have been 5 stars but it does have some issues with poor squadmate AI. Most of the time they perform well, but they will need constant babysitting to stop them from running off and getting into trouble in some missions. They also seem to have an annoying habit of crossing your line of sight just as you begin firing, this is especially noticable when using the sniper rifle. It's not game breaking, but it can be quite annoying and does detract a little from the gameplay.
Overall though this is a very enjoyable game, with plenty of depth in both storyline and gameplay.