Well, I've tried them. I can buy them at local stores. Usually the 3600 calories packs are around $7.
3600 pack
I also like to get the one that are a third of that (or equivalent to one meal, 1200 cal).
1200 package
As for calorie density, the 3600 packs are about 1.5 lbs, so it lines up with the normal target range of backpacking food.
I also have bought the Datrex bars. But, I have only tried the Mainstays and another brand that I can't remember. The Mainstay brand was better tasting. I haven't tasted the Datrex ones yet.
The two times I ate a Mainstay bar was at home, and once for breakfast whilst backpacking. I wanted to see it if had enough calories for me for the meal (I tried the 1200 calorie pack). The 1200 pack is split into three cookies. The first cookie was easy to eat, but the last one was hard to force down. It wasn't that it was gross, just really boring. However, I think the calorie amount was adequate, and it gave me enough energy to last till lunch time without feeling depleted.
For backpacking, there is a place for these types of foods. I will usually take a 1200 packet and just keep it in my pack for an emergency extra. I can also see someone using a 1200 pack as a meal addition. You take a third of it and add it to each meal. But it is all subject to the individual, and individual taste, and individual circumstances.