Personally I'm of the opinion that every go bag should include work gloves, a change of clothes, several pairs of socks, a bar of soap, a couple of flashlights, a couple of bandanas, a rain jacket, a mora knife and several pens including a sharpie, plus one of those Rite in the Rain notepads. I think water is good but I also include a dozen Eat-more bars and a liter of coke (caffeine and sugar is rocket fuel in an emergency).
I have other stuff in mine but I always had the thought: what if I had to grab it and run in a Vancouver earthquake? I'd want to be able to walk a long way, dig through rubble, change my socks a few times, go to a hotel, leave notes for people, and reduce my general level of misery.
I have a couple of stages of bags, starting with the above, plus a "what if I had to live out of a hotel for a month" bag.
I have to admit I haven't really revised it since I had a kid so I have a desperation bag that would keep me going for a bit, but I really need to re-jig it with diapers and wipes etc. I have a pre-stocked diaper backpack but it's a separate bag. I should redo my grab and run bag.