I've just come across a store that sells these a few weeks ago. So, like a good boy scout, I decided to test them. First, a description:
They're olive drab or day-glo orange cardboard boxes, about twice the size of a thick flod-over wallet. They are either sickeningly sweet (orange) or kinda nutty (OD).
What they are: These packs contain 210 grams of "compressed carbohydrates", divided into four indivudial bars, wrapped in wax paper, that can be broken in half. The texture is... interesting. Imagine a granola bar crossed with a dry cracker. Taste is not ecactly the best, but hey, it's food, and exceedingly compact. The green boxes hold 2500 kcals, the orange ones 1800, quite a lot for their size. They contain no water, according to the packaging a minimum supply of one liter per day is recommended when eating the orange ones, one and a half to two liters with the green ones.
The reason for this is that the orange ones are, essentially, pure sugar, with a little starch to dull the ingredibly sweet taste and to lengthen the effects. The Army says these were designed for low-water survival situations (ditched in the sea, desert, that kind of thing), and require almost no water to digest.
The olive ones contain about 15% simple carbohydrates (sugar), 60% complex carbohydrates(starches and such), 12.5% protein and 17.5% fat. 0.0% water. Due to the protein and the fat, these require more water to digest, but they provide about 2/5ths more energy than the orange ones. They also "fill you up" better, you don't get the feeling of being continuously hugry.
You won't find a best-before date on these, and the official word in the Army is 25 years. Inoficcially, it's "infinite", seeing as vacuum-sealed, freeze-dried food can't really go bad, especially if it's synthetic and as such free of anything except energy.
Results:
For most people, I suggest a mix, with two orange and a larger number of OD ones, The orange keep you going for two days until a water supply is established, then the green ones provide more nourishment. If your situation is very "dry", or ditching in salt water, take the orange ones exclusively. They are pretty awful to eat, but you can't beat the energy density, which, by volume, is more than six times that of a field-stripped MRE.
Personally, I've added an orange one to my daily carry backpack, increasing it's "survival stuff" load to 670 grams. Two orange ones and five OD ones are in the car.