I first saw this app about a week ago, and then I searching for a trial/illegal version to try... (price is a bit high for a flight-simmer)
But then this morning I decided to buy it and give it a fly.
After about 12 hours and 4 flights with FS9 and the iFly 737NG I can tell: this really works with Flight Simulator 2004.
I can't test it with FSX, so that's up to someone else.
Conclusion:
It works like they promised, and if you're as fanatic as I am, then the price is worth it. (especially if you've got an iPad)