I'm a regular user of cannabis (I smoke everyday after dinner, around 9pm or so). I frequently watch math/CS lectures, write code, and read technical books when high.
I find that it helps me a lot both with focusing, and with creative thought. Ideas flow more easily, bits of knowledge connect on their own. I've most certainly gotten productive work done while high. Recollection is not too much of an issue, but I recommend heavy use of a notebook to keep track of what you're doing, and so that you can refer to it the next day.
I've never used drugs other than weed/psychedelics, mostly because I find that stuff nasty and stay away from it, so I can't comment on that. It's close to impossible for me to get anything done during a shroom trip- however, when the trip ends, I enter a phase of extreme lucidity and am able to focus in a way that I've never been to sober.
Relevant details: I am a cofounder at a small but growing SF tech startup. I also believe I have Asperger's (I've always been that super weird kid) & ADD (I have a really hard time focusing and sticking to a single task, and the feeling of extreme focus and lucidity I experience at the end of a shroom trip feels to me like it should be the way my brain should always be), and that my brain chemistry behaves in a very atypical way with such substances (my gf has remarked several times that I am way more lucid and coherent than anyone else she knows on drugs) - I am currently in the process of getting those things professionally diagnosed. I've gotten my IQ assessed as a teenager, it was in the ~150 range.
Finally, the bay area is super open to that. I regularly smoke weed and talk CS theory/math with friends who work at large famous tech companies, and a certain subset of employees at a well known large tech company with a knack for design absolutely love the Ploom Pax [0].