Incredibly enjoyable.
Yes, the 9/11 attacks were a tragedy, but this game is still fun whichever way you look at it. It's been over 9 years now since the September 11th attacks, so to the people saying it's too soon, look at your calender.
The game was fun, not the best graphics I've ever seen, but you were pressed for time and even still you made it run smoothly enough that the graphics didn't even matter. Honestly, the only real reason people are complaining is the fact that "9/11" is in the game title. If this was called "Crash the plane" or something like that, nobody would care, I guarantee it.
Regardless, this game was awesome and it's not given enough credit just because people think you're a horrible person and a treasonous bastard.