Air Tycoon is great to start off with, but hits the wall very quickly.
But honestly, eventually all you are doing is building planes and adding routes everywhere and it quickly becomes boring and tedious. Your opponents dont seem to go bankrupt and even if you are much better than them and buy the company what are you expected to do when its just your airline on the planet. I understand there are limited capabilities for the Iphone/Itouch however surely events could occur, and having more cities and opponents would optimise the experience.
Air Tycoon is very much so a good game, great layout and simple to play, however like many games on all consoles it is short lived and although it seems to have great replay value, this dies out once you reach a certain point in the game. And eventually its just an app you dont really use.
Great setup, bad lifespan.