Factors why I have not and probably will not:

1. Soylent costs more than my current diet, limiting gains
2. it is a priori highly likely to fail since we know for a fact that severe nutrition deficiencies can be due to subtle & misunderstood factors (see: the forgetting of scurvy cures) and that nutrition is one of the least reliable scientific areas
3. his work is even more likely than that to have problems because he hasn't consulted the existing work on food replacements (yes, it's a thing; how exactly do you think people in comas or with broken jaws get fed?)
4. given #2, the negative effects are likely to be subtle and long-term means that on basic statistical power grounds, you'll want long and well-powered self-experiments to go from 'crappy self-experiment' to 'good self-experiment'*
5. given the low odds of success (#2-3), the expensive powerful self-experiments necessary to shift our original expectations substantially due to long-term effects and subtlety (#4), and the small benefits (#1), the VoI is low here
6. my other self-experiments, in progress and planned, suffer from many fewer of Soylent's defects, hence have reasonable VoIs (Specifically: I am or will be investigating Noopept, melatonin, magnesium l-threonate & citrate, coluracetam, meditation, Redshift, and lithium orotate.)
7. VoI current/planned self-experiments (#6) > VoI Soylent cloning/tweaking (#5)
8. hence, the opportunity cost of Soylent is higher than not, so I will continue my existing plans

* although see my reply to Qiaochu, at this point Rob isn't even at the 'crappy' level

EDIT: as of June 2015, I would amend my list of complaints to de-emphasize #3 as it seems that Soylent Inc has revised the formulation a number of times, run it by some experts, and has now been field-tested to some degree; most of my self-experiments in #6 have since finished (right now the only relevant ones are another magnesium self-experiment, trying to find the right dosage, and nonrandomized bacopa ABA quasiexperiment); and for point #1, between increasing the protein in my diet and official Soylent lowering prices, now Soylent is more like 2x my current food expenditures than 3x+.