They have relatively good drops for a medium level monster and are close to the bank, so this makes them a popular training location. As a slayer task they give 66 experience per kill. Note that you must have completed Lost City in order to be able to kill them.
They do hit high for their combat level, so care must be taken when training. They have a relatively strong Magic based attack, so wearing dragonhide armor will help a lot.
Looks pretty otherworldly to me
Dragon med helm
Coins (1-420), Cosmic rune (2), Law rune (2), Nature rune (5-10), Chaos rune (4), Death rune (2), Blood rune (2), Ruby ring, Mithril mace, Grimy herbs, Mackerel, Nature talisman, Battlestaff, Rune javelin (5)