Is there any advantage of using fly ash as supplimentry cemeneting material in concrete?

Yes, the addition of Fly ash will decrease the cost of product and it will further increase finess of the blend (depending upon finess of fly ash) which will result in more work-ability and later strength.

Yes, it improves durability properties of concrete, basically presence of fly ash results in prolong hydration producing calcium  silicates and reducing CaOH, which results in more durable concrete. In addition to this replacement of certain % of cement with fly ash, increase the workability and reduce the cost of concrete.