Half Life 2 is a first person shooter made for mouse and keyboard controls, which can be ported to a controller if needed. For Natal developers will have to think outside of the box, a FPS could be possible but not in the traditional sense. Perhaps with less extreem movement and more dodging. Or perhaps with more auto aim, where you simply point and stretch your arm to shoot and the actual hard part fo the gameplay is the dodging and not the shooting. For that I would but Natal because I'm tired of FPS that are just about pointing and shooting.
Natal could still be a succes but Microsoft has a lot of work to do, they need to make it interesting for developers that are currently making 360 games and they also need to convince Wii developers to make kids games for the 360. Only if Natal is accepted by both mainstream and hardcore gamers will it be a succes and actually help the 360. Because if it doesnt attract the mainstream gamer then nothing is gained as the 360 is already a succes among hardcore gamers.