I used 2cd at 10mg a couple of semesters ago while studying for astronomy finals. I've heard others mention, and found true with myself that it did help me study, slightly boosting my retention because it seemed to 'quiet the background noise'. Strange enough, the slight motivation I felt was easy to channel into the task at hand.
Mind you I only tried that once, so cannot make a truly empirical conclusion.
In light of this thread reviving my interest 40mg a piece were sprinkled on the brains of two willing participants who both had a great experience. It was stated that 40mg 2cd was more enjoyable than 35mg 2cb, and was not even close to being overwhelming.
That said, I probably would not use this as a nootropic again; rather, I would save it for a time and situation more special to its carnival nature: pinwheels and calliopes, windmills and galaxies all spinning around a white hot tiny singularity. I can only imagine the secrets that 2ce or DOM might unveil.