How dangerous are volcanoes?
Volcanoes are usually less dangerous than other natural hazards such as earthquakes, tsunamis and hurricanes.
But there is no good answer if you don't limit it into a specific context: which volcano? dangerous to what - people, property, etc.? during which type of activity? at which location?
Volcanoes have a serious of hazards (e.g. lava flows, ash fall, pyroclastic flows, climate changes on a global scale) that relate into different dangers or risks. The risks when visiting an active volcano depend on which risk zones of the volcano are visited and for how long. More on volcanic risk and risk zones...