Yep, I thought it was a joke at first, too.

Since the term soylent is widely understood to mean human remains masquerading as food, the name suggests pretense.

He wrote:

    Before I rarely had enough energy to go to
    the gym, but this day I had plenty so I 
    decided to put the diet to the test. I'd been
    running off and on for several months, never 
    able to do more than a mile straight, but
    this day I ran 3.14 miles non-stop. This is an
    irrational improvement.

!! Indeed, it does sound irrational, doesn't it? In fact, whether this is a hint or he's just being cute, I literally don't believe that Soylent caused this uptick in performance. Want to bet on the results of a double blind study?

Elsewhere, he said he only needs 1/3 the calories on Soylent, which was explained away by an apologist in the comments as a figure of speech, or perhaps art of rhetoric, but that explanation is contradicted by his precise descriptions elsewhere and worrisome in this context regardless.

There were more jokes like that, conveniently allowing its author to laugh at people no matter whether he's being truthful or not. Parts of the campaign are patronizing and condescending and they've shown no desire to fix that aspect, making me more leery due to the immaturity. Should I trust my health to such a cavalier attitude?

I lean toward it being legit, but they've got a long ways to go to catch up with the cutting edge of understanding in nutrition science and it's not clear what advantage they'll be able to offer over established meal replacement drinks like Ensure, which most people don't want to drink today. Improved taste? You can get rats to overfeed to fatness on chocolate Ensure. Expertise? No. For personal use, Soylent Orange is more convenient, available today, and safer, IMO:

http://lesswrong.com/lw/h2h/i_hate_preparing_food_my_solutio...