We had air suspension (rear only) fitted to our Premium Van from new (Goldschmidt). Not quite a tag axle, but successful in itself. Unfortunately I cannot compare with leaf springs only as I have never driven without the air suspension. We find an additional use which is to give us greater clearance when entering/exiting our drive, which is a steep incline off the road. I pump up the pressure to ensure the towing hook clears the road.