As a former chronic smoker, the only negative thing I would say about weed is that it DEFINATELY ZAPPS MOTIVATION. Anybody that tells you different is out to lunch, IMHO.
I'll argue this. I know plenty of people who get high as fuck and then look like the most motivated person you've ever met. The trick is, they're doing something they WANT to be doing. Now, if you're about to do something you DO NOT want to do, then yeah, getting high will sap what little motivation you had to do it. Being high just provides an excuse to be a slacker and put off something that you don't really want to do anyway.