Sierra is the same company as Peak, good stuff. You're more likely to have corrosion introduced by adding tap water, rain, remaining water flush, etc. than a breakdown of the product if you are keeping regular maintenance intervals. The only over-priced coolant I would consider is a non-aqueous propyleneglycol (http://www.evanscooling.com/main27.htm) but I've never had the need. Just keep in mind that Engine Ice (homeade or not) is mixed with de-ionized water, so if it is boiling over you may need to move to a pure coolant.