Not being a partaker, this news is of theoretical interest only:

Forget the stereotype about dopey potheads. It seems marijuana could be good for your brain. While other studies have shown that periodic use of marijuana can cause memory loss and impair learning and a host of other health problems down the road, new research suggests the drug could have some benefits when administered regularly in a highly potent form. Most "drugs of abuse" such as alcohol, heroin, cocaine and nicotine suppress growth of new brain cells. However, researchers found that cannabinoids promoted generation of new neurons in rats' hippocampuses.