I play guitar and mandolin. I have a jam band with some buddies and smoking before a jam session is pretty much commonplace, and we can usually come up with some pretty ridiculous shit. So i would have to say it benefits my creativity and improvisation skills, it just gives you a whole new perspective to your playing, plus a different perspective of what all the other musicians are doing, thus creating new opportunities.

But alcohol and cocaine just make our playing go absolutely wild. some of the best jams we've had have been because of good old alcohol and cocaine. Just gets you in the groove.