I was 21 and remember it pretty well - I still have a lot of questions about that day that don't seem to add up or make sense really, from a military standpoint and with regard to some other things that occurred that day (collapse of WTC 7, Flight 93 most likely shot down, etc).
Say what you will about "conspiracy" theorists, but lots of unanswered questions and things that just don't make sense. I'm not saying I believe anything really to the contrary of the "official story" (well, what was actually put in it - omitted WTC7, Pakistan connection that has now come back around again with Bin Laden's killing), but just many things were out of place that day.
Regardless, it was a extremely horrible and inhumane incident and something that has truly changed our nation and society - in both some good ways (unity) and some bad ways (loss of some freedoms -debatable on a case by case basis). My company lost a lot of people that day, and the first responders and military men and women who gave their lives that day and for the last 10 years on, have given the greatest sacrifice.