Wow now this is an interesting thread, I've just spend the last hour or so reading through it!

Thanks for bringing this up Andy.
Now I have a similar quest for knowledge, or to be more accurate wisdom because as you say, there is no right or wrong. As Socrates once said "The only thing I know for sure, is that I know nothing". My quest began due to smoking (actually eating first off

) cannabis - viewing the world from another perspective leaves more questions than answers.
My observations
I have all but given up on scientific evidence, largely due to studying philosophy at uni. Instead, I tend to sit on the fence and just observe. What I have observed about cannabis includes the following:
- Thoughts are either sped up in my mind, and/or MY ability to observe thoughts is diminished and/or time itself is effected (the latter being unlikely!).
- Cannabis should only be smoked regularly by a strong, or psychologically trained mind. It is easy to let the drug take over my thoughts.
- If I enjoy the feeling of cannabis too much without keeping my mind ticking over, my memory suffers.
I have used a first person perspective but predict that the above three points are generally "true" for everyone.
Cannabis / madness
Here is a link to a subjective study of cannabis carried out by a mystic known either for his genius or wickedness:
http://users.lycaeum.org/~sputnik/Ludlo ... psych.html
Here are two excerpts from this work:
COMPARABLE to the Alf Laylah wa Laylah itself, a very Tower of Babel, partaking alike of truth both gross and subtle inextricably interwoven with the most fantastic fable, is our view of the Herb -- Hashish -- the Herb Dangerous. Of the investigators who have pierced even for a moment the magic veil of its glamour ecstatic many have been appalled, many disappointed. Few have dared to crush in arms of steel this burning daughter of the Jinn; to ravish from her poisonous scarlet lips the kisses of death, to force her serpent-smooth and serpent-stinging body down to some infernal torture-couch, and strike her into spasm as the lightning splits the cloud-wrack, only to read in her infinite sea-green eyes the awful price of her virginity -- black madness.
and:
The fear of being swept away in the tide of relentless images is a terrible experience. Woe to who yields!
If you are interested in the real effects of cannabis, I recommend reading this and following the links in the text. It took me a while to deciver his complex and sometimes poetic language, but from my experiences, I largely agree with his analysis on the psychology of hashish. Given a chance, cannabis can lead to psychological problems or "black madness" as Crowley calls it. I have seen many of my friends lose the plot, however, many have also smoked copius amounts of hash with no negative effects. I believe this comes down to the will power of the smoker to keep control, in a similar way to the drunk becoming agressive or keeping control.
Cannabis / anxiety
From a personal viewpoint, there was a time when I would suffer from extreme anxiety if I smoked a lot of cannabis, which made me wonder why it happened to me and not others. After intensive meditation I realised that a lot of my unconscious processes were being left unchecked, including unreasonable fears and rediculous conclusions.
As Soulrider mentioned, the increase of sensory information caused by cannabis can be overwhelming and cause confusion to your perception of surroundings. Unless you train your mind to filter through your thoughts and feelings whilst high, it is likely that you will become confused!
Cannabis / Depression
With regard to depression, I now do not believe that cannabis causes it. Yes ecstacy does, but a lot of cannabis smokers take other drugs and may be lead to believe that it's the cannabis which is to blame. I have stopped taking all other drugs for a few years now (except the odd shroom session

), and whilst still smoking cannabis, I no longer feel depressed.
Cannabis / memory
On a last note about memory, I recall reading another subjective study carried out which suggests that cannabis can actually be used to reinforce memories. It helps to "burn" memories into your mind so that they are harder to forget. I'll add the link to this if I manage to find it again.
So thats what I've learnt about cannabis off the top of my head. I believe it has a lot more to offer with regard to exploring the abilities of the mind. Crowley noted that during his visit to India, he noticed that the yogis were using cannabis to assist with their meditation.
Yes it can impare memory, but only if you choose to let it do so. It's not the drug which is the problem, it's the choices you make

.