Music is good for atmosphere but ambience alone is better in my book. A good example of this is the Thief series, which is why I think it is one of the most atmospheric games ever created. The ambiance sounds were right on point. The main problem with Music is it can sometimes hurt the mood if it doesn't fit with whats going on in the game world, becomes too repetitive, or just doesn't sound good. And since its more about context, the experience generally varies more from person to person.