Lo-salt substitutes are often made with potassium chloride (rather than sodium chloride). Certain medical conditions and medications make it harder for the body to eliminate potassium, thus taking in extra potassium in an attempt to avoid salt can be harmful. Very often these are precisely the conditions that might cause someone to look at reducing salt intake in the first place (kidney disease, for example, or certain diuretics - taken for fluid retention problems, to which high salt intake is thought to contribute), meaning that good-intentioned but underinformed folks with these conditions can end up doing themselves more harm than good.
Personally I don't worry much about salt, I do salt my food, some foods quite a bit. BP is fine, kidneys are fine, so I don't stress over salting my food. Anyway salt we add ourselves to food is usually pretty minimal as compared to salt that's "built in" to a lot of foods. Cutting processed foods - especially things like canned soups and sauces, commercial bread, processed meats and the like - helps in two ways. Not only does it get rid of all the salt that's in the processed foods to begin with, but not eating them leaves your taste buds less used to the taste of salt generally. So when the time comes to put salt on your food, you're more sensitive to the taste and need less for it to taste "right".
So if I were concerned with sodium intake, I would focus my energy on processed foods rather than on salting food less or on replacing table salt necessarily (unless you want to of course; there is something to be said for *seasoning* food and not just salting it!)