There does seem to be some pretty persuasive evidence that mj has a number of beneficial effects, but even in the tests results and posts that our most enthusiastic test-citers never fail to use in support of weed, there haven't been any huge or long-term studies proving overwhelming, without-a-doubt benefits. Just some very promising positive evidence that needs to be studied in more depth and over a longer term.

One correction to something LIP posted above. Cannabis doesn't kill glial cells or gliomas (glial-originating cancerous brain tumors). In the studies that have been done, THC seems to decrease their growth and help inhibit the vascular formation that feeds gliomas. That's a really big difference from outright killing those cells. There are some interesting studies going on in this area, and more need to be done.

Also, LIP, just as an FYI, there are lots of different types of brain cells that become cancerous, not just glial cells. Among them are neurofibromatomas, ependymomas, meningiomas, adenomas, astrocytomas, oligodendroglial cancer cells, medulloblastomas, pineocytomas, parenchymal cell tumors, craniopharyngomas, germ cell cancers, and several others. So far, the main one we know of positive effects from THC on is the glioma (glioblastoma multiforme), which is common and very deadly. Maybe in a few years, THC will be used routinely on these.