>I agree on balance people should generally play the numbers game, not the custom cover letter game. But, if there's a particular job you really want, it might help.

If there's some job or some company that you're really interested in and especially if you have some network there that might give you an "unfair" advantage, by all means go above and beyond to maximize your chances at that one company. But I agree in general that, for most people--especially more junior people--it's really a numbers game. That's not to say you shouldn't do little things that might make you stand out a bit for a given employer, but you should mostly be casting a wide net.