I use cannabis for creativity and motivation, in both writing fiction and playing piano. I can't prove that it gives me better ideas, but looking back from a state of sobriety, I believe that it does, and certainly I've been more productive high than I ever was sober.

The catch is that I take frequent breaks. My current rule is, for every session, I have to go one calendar day without using it at all. This also keeps my tolerance low, so I only use about a gram a month.