The problem with the word 'healthy' is that if you're trying to discuss nutrition rigorously, you really can't call a given food (or substance) healthy. It really only works as an adjective for an entire diet. If someone already consumes way too much vitamin C, more vitamin C is definitely not healthy. And yet for a sailor who hasn't seen greens in months, it definitely is. So, nothing can really be seen as healthy in isolation, the context is essential.
So then, the correct question is "Is a diet of only Soylent healthy?" Even here we run into some pretty significant trouble: nutrition is not a developed enough science to really answer that question yet. We don't have a good metric of healthiness, even for an entire diet. What we can do, though, is make comparisons. We can ask if a diet of only Soylent is healthier than a diet of only rocks. That one's a pretty easy yes. More meaningfully, we can ask if a diet of only Soylent is healthier than a diet of only fast food. I'm confident that it is, but this is where the disagreement begins. Unfortunately, this stuff is hard to test. Forcing someone to eat nothing but fast food for an experiment would be completely unethical, and observing people who already only eat fast food would be subject to some really significant sampling bias.
This has all been a long-winded way of saying that there's no way to know, yet. I would strongly argue that adult consumers should be free to experiment with their diet as long as they do it responsibly. There are definitely some moral and legal gray areas here ('experimenting with your diet' by downing a bottle of pills is still attempted suicide).
I think it's highly likely that a diet of only Soylent is quite a bit healthier than the diet I'm currently eating (way more fast food and take out than I should, for a variety of reasons). That being the case, I'm very happy to give it a try.