All scorpions are venomous. However, the venom of emperor scorpions is not considered life threatening, unless you have an allergic reaction. Their venom produces a painful reaction that has been equated to a bee or wasp sting, and stings should obviously be avoided. They are also reported to deliver a painful pinch with their pedipalps, so overall this is a pet that is better suited to watching, not handling.
Other scorpions can be much more dangerous (some are venomous enough that a sting is fatal to humans), so your best bet if you want a pet scorpion is an emperor scorpion.
Emperor scorpions can make good pets. They do not take much space, are not noisy, and are clean creatures. As long as you provide the proper heat and humidity and a supply of insect prey, a pet emperor scorpion can live several years.
More on Emperor scorpions.