To quote Crocodile Dundee,
…Well, you can live on it, but it taste like shit.
Yeah, they have gotten better in taste, if you find the ones you like.
They are very high is sodium. And you have to drink a lot of water with them or you may experience a bowel blockage. I seen it more than once out in the field. I fully expect to see/hear about some survivalist who has a years worth of MREs to die of a bowel blockage in a post SHTF world.
They are not meant for long term consumption. It is kinda like eating fast food, and super sizing it everytime.
Post SHTF, I would not be surprised to see someone willing to trade a case of MREs for some fresh food.
Concerning water, the heaters that come with them, you can use nearly any water to make them work. Work better with clean water than using mud, but you get the idea.