You're right, of course, that there is evidence from experience, and other comments discuss the extrapolation of such evidence to Soylent, both for and against its risk to health.
I mean rather to address the calls for randomized, controlled experiments. Indeed, I would like to see a rigorous clinical study of Soylent, and I would not believe any specific claim about the health benefits of Soylent without at least that much evidence.
But the standard employed by most people for making dietary changes, even major ones (e.g., any fad diet), is basically whim and fancy, and perhaps that is not unreasonable given the low cost of making these changes and the difficulty of obtaining clear scientific results about diet (exacerbated by large interpersonal variation).
I'm surprised by the strongly negative reactions to this. Yes, the claims being made about Soylent are ridiculously overstated and undoubtedly will be softened with time. And yes, I suspect that some ill effects on health will result for some who subsist entirely on Soylent, especially in this first public version.
But I also suspect that very few people (and only those prepared to accept the consequences) will attempt to subsist entirely on Soylent for long periods of time. What I think interests most people is a way to recover most of the time they spend eating and thinking about food, while enjoying regular meals when it is convenient to do so (perhaps once daily before leaving the house or during lunch with coworkers, or a few times a week when eating socially with friends).
Diets that are mostly made up of a few ingredients aren't news; they're the way most of the world eats (see, e.g., staple foods). Soylent attempts to reorder the diet to include long periods where eating has low prep cost and relatively high nutritional value, instead of the status quo which has lots of meals with moderate prep cost, moderate nutritional value, and the unfortunate side effect of breaking up non-eating time into small blocks.
Of course there is no evidence that Soylent provides everything you need to be healthy, or none of the things that will make you unhealthy, just as there's no evidence that your current diet does the same.
But that's okay: Soylent is, as many commentors have pointed out, neither the first of its kind nor a scientific advance. It's a marketing advance. Before Soylent, a diet consisting largely of a single liquid brought up associations of illness and weight loss. It doesn't sound like something you would want to try or tell other people you were trying. The Soylent diet, on the other hand, you associate with health, saving time, and munchkinism. If Soylent succeeds, its success will be in overthrowing the three-meal-a-day status quo. As someone who would love to spend less time thinking about, pursuing, preparing, and eating food, that's an advance that I welcome.
1 is based on assigning what value to your time?