It's funny. If you like his kind of humour. If you don't like his comedy, as some reviewers do not, then you won't enjoy it... Simple as that. I've never seen him before but he made me chuckle a few times.
To expect a serious medical experiment is ridiculous, it is only 1 individual for one month so the effects would never be statistically significant. To take the doctor's findings seriously is to miss the point.
The film was more about legalisation of cannabis than of the effects of smoking a lot of pot and it was a humorous way to bring attention to an interesting subject.