Northwest Airlines is not necessarily known for its outstanding service. They especially have to work on the friendliness of many of their employees and on their baggage handling (No other airline has lost my luggage as often as Northwest. They are also known for refusing compensation when your bag gets damaged).
However, Northwest is usually cheaper than its competitors and they continue to serve many smaller markets with full size aircraft, which is a big advantage over certain other airlines that serve small and medium markets with regional jets. Also, if you get to fly on their new A330 on the longhaul, you will have a personal video system and other amenities that you might have seen on more expensive carriers. Of course personally I don't like the 330 for its rather tight seating, but that's another story.