I read a small review of a study once... they tested aircraft pilots from major airlines.

They tested their reflexes under
A- normal
B- caffeine
C- THC

I don't quite remember how they supplemented the THC to the pilots... I guess it was some sort of controlled-amount pills or something...

Anyways, pilots with cannabis in their system where better than the caffeinated pilots, and better than the normal pilots.

I remember the lead-researcher's thoughts on the subject: In essence, he would rather have a cannabis-using pilot than a caffeine-using pilot.