IF#1 and Epsom Salts work in entirely different ways. IF#1 contains stimulant herbs and herbs to loosen and lubricate. Epsom Salt works by dumping a large amount of magnesium into the system forcing the body to draw a lot of water into the bowel to flush out the excess magnesium. It is more of a flush than a true cleanse, also I guess with continued use any hardened matter lining the bowel would eventually be softened enough to loosen and pass through.
If you are constipated due to lack of peristalsis and poor muscle tone, then a limited cleanse with IF#1 or a similar product would probably be best. If you usually have daily movements and just need a one time flush, then Epsom Salt is a good way to go. The most natural way is to drink plenty of pure water, eat high fiber fruits and veggies, do deep breathing to get plenty of oxygen in the system and exercise to regain muscle tone. Cleanses are good to repair damage already done, but ideally, our goal should be a healthy naturally functioning colon that needs no intervention.