I'm allergic to milk and have been taking soy protein, soy milk, etc. for years with no problems. My evidence is anecdotal but silverhydra's is not.
If anyone thinks there is a problem with soy, go read examine.
It seems to me people are compelled to have a milk substitute whether due to allergy or ethical reasons as if we need to have something called milk in our fridges.
It reminds me of the tofurky a soy shaped turkey supposedly tastes like turkey, it would be like a person who eats a lot of meat making a carrot out of meat colouring it orange and making it taste like a carrot, why even bother.
I used to do soy protein + water and it was pretty bad but I could handle it. I recently switched to Beef and Egg protein from Soy protein and it tastes amazing in chocolate soy milk.
When you can't eat cheese, most desserts, pizza, and anything with a questionable production practice ("did my food share a surface with a milk product?"), and much more, you eat things without a lot of taste. I like Soy milk because it tastes good and keeps me sane while getting me my calories.
Some folk have an ethical objection to milk but want something that vaguely resembles it. For such people, whitened liquid cardboard -- soy juice -- is a popular alternative. Other people drink almond juice or rice juice.
Edit: and some people have allergies or intolerance to things in milk. I'm lactose intolerant, so I'm covered by lactose-free milk. But some people are genuinely intolerant of milk proteins etc.
whitened liquid cardboard
I found it tasted, quite sincerely, like cardboard. I was reminded of the Adams line about "almost, but not quite, entirely unlike tea".