Somewhat to very. Marijuana is no longer the mellow 70s drug that it is frequently referenced as. Growers have bred it to be many times as potent and sellers even lace it with other chemicals (bleach, formaldehyde, cocaine) to enhance the experience. One is bound to experience withdrawal and long term sequelae fom such product.
Subtle. There are subtle personality effects in long term mj users, along with the effects of smoking (e.g., immune suppression, decreased lung function, increased cancer risk). Of sharper concern is that a substantial number of people with mental vulnerabilities may have their conditions worsened (depression, bipolar, schizophrenia) and people with sub-clinical conditions may be provoked into overt disor.