Yes, I think it is well worth it especially when you are departing Iran. And yes, they have their own duty free section and nice toilets (farangy) and very clean which makes it even worth more.
Basically, when you enter Iran, they are waiting for you with a sign right outside of the plane (not the gate). They will then take you with a car to CIP terminal. You will hand them your documents and will go and relax in a very nice lounge with lots of good food and drinks (free). ~30 minutes later, someone shows up with your passports and suitcases and ask you if you need a car or not.
You practically don't talk to any Immigration officer.