It was a master stroke and well done.
Just wanted to add my two cents-
I seems to me that the most important part of this movie is not where to place the blame for 9/11 but what must be done in the future and how vital it is we do it to prevent another series of attacks. Throughout the movie, there were many many subtle but noticeable messages about what really went wrong. Among others, it was clear how easy it was for the hijackers to get inside the country. There was the FISA situation where they didn't feel they had enough evidence to go to the court. The not-so-subtle Gorelick Wall...the list goes on.
I know I'm gonna take heat for saying it, but I truly don't blame Bill Clinton for 9/11. Could he have stopped it? Likely. Should he have tried harder to stop it? Without a doubt. 9/11 was truly a failure of imagination, and I'm as guilty of the sin of not reading the writing on the wall as everyone else was.
The important message to walk away from was not where to point the finger (though I feel it did, and justifiably so) but what to do in the future. I would define this movie as successful not if it scored political points for the president (and I hope it will) but if it serves as an impetus for people to become serious about fighting the war on terror.