A good user interface can make all the difference. It can turn a decent game into something altogether more or make a great game look bad.
A Masters thesis from Chalmers University of Technology created at DICE in 2009 by Magnus Lorentzon and Erik Fagerholt has surfaced, specifically covering ‘User interfaces for increased player immersion in FPS games’.
The document is a detailed look into the mechanics and psychology behind the HUD, and then some, covering roughly 20 games on the PS3 and Xbox360. It’s an interesting read and a great reference, so check it out.