Huge money eater, you will never really finish a challenge (unless you pay for it) Hatching and breeding even uncommon class dragons can take days (unless you want to pay for it) and even topping up gems and food become almost pointless (unless you want to pay for it) and if you manage to complete a offer but get no reward, don't bother trying to contact devs they will not help you. There are much better farm games than this out there.