The "stealth" aspect (as Ira put it) is important, because I really don't want someone looking into my car and seeing a bag that shouts "Expensive Camera Equipment" sitting on the passenger seat. I always have a cheap jacket in the car, and will casually drape it partially over the bag if I've got to leave it for any length of time.
For stealth bags apparently diaper bags are favoured. Let's face it, who is going to steal a diaper bag. Insulated lunch bags are also good but someone might think you have beer and steal it however using the insulated bag for shock protection inside a cheap schoolbag type backpack would probably work. The best rule of course is to not use any camera bag with Pentax, Canon, Nikon, Olympus or even Minolta printed across it in bright letters, that is like saying "steal me".