I felt trapped in an awful PhD program (a well-known physics dept in the US) with an extremely toxic advisor. After years of getting yelled at for small mistakes with little or no feedback, insane hours (weekend/holiday meetings and deadlines) and zero growth I had completely lost any confidence. I became depressed, overweight, afraid to talk to people, etc.
I'm weary of drugs, even caffeine, but microdosing helped me regain some sense of direction and a bit of confidence to move forward. It is like simulated annealing as gwern describes. My mind was trapped in a minima and I needed a chemical shock to move it elsewhere.
As a side-note, I finished my PhD and am job hunting. Now, I'm exponentially happier around friends and family. I'm still rebuilding my confidence but I've stopped microdosing because I don't feel the need.
I've never taken a regular dose.