Kilz does work, especially if you are covering up mold or a greasy stain, 1 or 2 layers of Kilz will keep the stain from coming through as you pain over it. Also great to just use as a base layer although it has a strong odor to it when it is first applied.
Kilz paint is a little more expensive, but it is well worth the extra few bucks to cover and hide stains that would normally bleed through. Water stains can be awful to hide, but kilz covers very well.
Kilz will cover mold stains and stop mold growth as well as preventing the stain from bleeding through when a wall is repainted. For dark stains it's better to apply several thin coats of Kilz (waiting for each to dry before applying the next) rather than one heavy coat.
Kilz paint works great, especially on areas of your house that have problems with mold. It is best to clean the area before hand and let dry before applying Kilz paint. They also sell odorless Kilz paint which is great to use on the inside of the house.
It depends on the application you are referring to. Most of the time they seem to work great. I've used primer on several different projects and they always turn out really nice. It is highly recommended at most hardware stores.
Yes it does. It is a thin primer, almost like water. It can be hard to work with. Kilz looks like nothing is going on but it blocks odors and stains when dried. It can be messy so a drip cloth should be used, along with a fan.