The little two inch(?) ones are ideal for carrying around with you. They've got a knife, scissors, a screwdriver, a nail file, and tweezers and toothpick. The only thing i wish it had that it doesn't is a bottle opener. Larger swiss army knives definitely have cool gadgets but the utility value doesn't climb anywhere near as fast as the size/mass does. There's no way i'm going to carry one of those twenty tool monster knives around in my pocket. Even the medium sized six or seven(?) tool ones are bulkier than i want cluttering my pockets, though i do own one which i leave lying about the apartment.
Unfortunately the corollary to "small and easy to carry around" seems to be "small and liable to fall out of your pocket without you noticing" =P
So now i need to go by REI and get yet another new one. I reiterate what i believe i've said before, it would be damn convenient if they sold them in six-packs =P