I think ganja shouldn't be used in yoga classes when you are there to learn fro a teacher. As it can be misinterpreted by the other people if they notice this from you, unless you have a very specific reason for doing so.

In your personal practice though, I can advice anyone to give it a try. I myself do it from time to time and when I am able to stay concentrated it will put me in a really deep flow state when I go through my asana practice, and my pranayama will become meditative.
Also my session would often take more clock-time when I do this as I put in a lot of poses where I can go deep into myself (think of yin stretches, long sarvangasana and sirsasana holds).