Just popped in here with my son as we're staying in Selfoss while touring the ring-road. 3600ISK bought us two burger, fries and Coke meals. Service is fast and burgers come with all the trimmings. Burgers were cooked properly, and still pink in the middle.
When we left, the chap gave my son a free ice cream, a nice touch :)
A good place if you fancy a simple meal at a reasonable price.