A medicinal herb it is also an invasive species that is listed on the noxious weed list in over 20 countries. In pastures the plant can cause a lot of trouble by out competing native plants and can potentially poison cattle and livestock who eat too much. Here in the west we have 3 different beetles that have been introduced as biological control agents. WSU has a test field near Kettle Falls that is used to test various biological controls on the noxious and invasive species.
Medicinally St. John's Wort is used primarily to treat depression BUT has dangerous side effects from interactions with other medicines or drugs. It reminds me of the large problems with MAOIs and their interactions with almost everything.
Rhizomous plants can be extremely difficult to eradicate since just a piece if the root can regrow the plant. If the plant is contained it will work well in most gardens, but caution is advised when looking to plant in the ground. Properly preparing the planting site with root barriers can reduce a lot of headache later on.