That depends on the enamel finish. If it hasn't been ruined then you shouldn't have to use harsh abraisive cleaners like comet or even vim. Sounds like you have an old tub or a tub with a worn finish. When the enamel is worn off it looks dirty instantly and is very difficult to clean. When the enamel is good, you need only use a mild cleaner, even shampoo will get rid of any soap scum with a cloth.
You might want to look at having your tub refinished if its really bad. If its not bad, try baking soda. It works well at removing soap scum and other things that make the tub dirty plus there is no harsh chemicals.