I didn't go through the link that The Viking shared, so if this is a repeat, I apologize.
In an effort to save weight and dollars, I've only bought the entrees (at a much reduced price) and supplimented with home made parched corn flour, rolled oats, honey, coffee/tea and a few spices/condiments. Their taste is pretty good and instead of the heaters, my preference was to heat them in water, which I used later for cleanup.
They will keep a long time if stored properly. If you keep them refrigerated, they will last the longest, but the warmer the storage temperature, the shorter the serviceable life. IE if you store them at 120*F, they last a month. If at 60*F, it extends to 84 months. If the storage temperature varies, it will affect the life expectancy of the MRE. IE, store them at 100*F (18 month life) for 11 months and move them to 70*F (55 month life), their life is shortened to half of the 70* life.