Speaking from past personal experience and ongoing observation I would say that the effects of "dreadweed" would be predominantly negative.
The effects can also vary from individual to individual and, for the same individual, from one experience to the next. Though a given person will generally react in a predictable way to being stoned.
Common short-term effects would be:
1) User loses inhibitions - becomes more social (Cha may improve). In some cases, the feeling of mild euphoria accompanying this kind of experience can lead to delusional thoughts - the user feels super-intelligent and charming when in fact he or she is acting like a drivelling idiot.
2) User becomes introspective and disengages from environment (Cha drops).
3) User suffers "stone paranoia" - Why is everybody looking at me? Oh my god, I must look like I'm off my face. In some cases this can progress to a full-blown panic attack.
In all cases, Int, Dex and Wis would all *drop*. Cha may improve or drop.
As someone mentioned earlier, there is a great deal of anecdotal evidence to suggest that regular use of marijuana can trigger psychosis in some individuals. Schizophrenia was mentioned, but another common experience (and mine) is the development of a Panic or Anxiety disorder. However, studies on the subject are rare and inconclusive. It may be that people who are prone to certain types of mental disorders are drawn to marijuana in a subconscious attempt to self-medicate.
Long term effects on occasional users seem non-existant. Most regular users become increasingly socially withdrawn, but some don't seem to suffer any long term effects.
I used to work as a sound engineer and noticed that the effect of pot on a musician's performance was variable. Generally they all *felt* that it helped their performance, but in fact it would detract from their playing as often as it would improve it. Sometimes it appeared to help them hit the groove, other times it sent them all over the shop. My rule of thumb (and advice to my clients) was that they should never go on "stoned cold" - they should always wait until after their first set, or a good warm up session before lighting up. In D&D terms, this would translate into a penalty on Performance checks if the PC hadn't warmed up properly, a bonus otherwise.
A shamanistic prestige class specializing in divinatory magic involving drug use could very well work. It could certainly make a very good NPC class
