A friend of mine is a staff economist at the FDA who sometimes works on nutrition-related issues, so I asked him for his thoughts on Soylent. This is what he said:
This could be a useful product for a lot of people. It would probably be an excellent emergency meal, and is clearly better than most types of fast food or protein shakes. But do not forget that it is basically just protein powder, grains, and chemicals. If you tried to live exclusively on it, bad things would probably happen. Bioavailability is important, There are a lot things that are good in their natural form but worthless in supplements like this. But if you lived mainly on this and also ate a salad a day along with it, you would probably be fine.
For context, I should mention how science discovered and defined the essential vitamins and minerals. They are things that cause obvious and damaging diseases if your diet lacks them over the short to medium term. I suspect that there are many other semi-essential things whose absence could cause small problems, or large problems over the long term. Soylent starts with simple stuff and adds the known essentials. If you tried to live on it for years, you would probably develop a new and interesting nutrient deficiency disease.