I haven't been spicing up Soylent lately, but I'm no longer on Soylent 100% (or close to it). I'm fine with plain Soylent as long as I'm getting flavorful meals at other times during the day. When I do add flavor... cocoa all the way.
I don't remember if I mentioned before, but I was getting depressed on the 100% Soylent diet, and I kept craving spicy food. I realized it was the lack of stimulation bringing me down. Adding spices was adequate to keep my mood stable, and dessert type spices go especially well with Soylent's... I don't want to say palate, because it's purposefully bland... texture? It's very much a milkshake.
My eating habits now are: whole foods when I have plenty of time, Soylent when I'm on the go. My lifestyle has been a comfortable pace lately, so I'm mostly eating whole foods. It just... feels better. For me, Soylent excellently satisfies the "survive" part of "survive and thrive." Thriving for me is worry-free, zen-like preparation and consumption.