Second, it is a fictionalized docu-drama. It is not a straight-up documentary retelling everything that happened exactly as it did. It does mix in a lot of real-life news footage of events that the series depicts and footage from the congressional hearings that were held a few years after the attacks. Some characters were a combination of real-life people or a fictionalized version of real people (like the Schmidt character (played by Peter Sarsgaard), who was based on a pretty crazy real-life CIA agent at Alec Station), and some events were changed a bit (such as the bombing of the USS Cole). So, if you are looking for something that gives a straight-up retelling, then this is not it. Nor does it put a ton of emphasis on 9-11 itself. The attacks and aftermath mainly play out on monitors playing real-life footage in the background in the final episode. There was very little that the show filmed depicting things that happened on the actual day itself. What was filmed were things like Richard Clarke in the White House as it was being evacuated, and later in the bunker, one of the FBI agents walking through the streets after the collapse of the towers, etc. A big chunk of the last episode involves Ali Soufan finally getting access to interrogate Bin-Laden's former bodyguard who was being detained in Yemen and distills what was an interrogation over several days into a 10-or-so-minute sequence.
The acting and writing of the show are very good. Jeff Daniels really shines as John O'Neil, and for all but one episode is pretty much the main character. Tahir Rahim also does a great job playing Ali Soufan, who was part of a group of FBI agents butting heads with the CIA to get information. Some of the real-life players were also consultants and/or producers of the show, which I think helped its authenticity. For extras, there are commentary tracks on the first and last episodes by the writer and director of the episodes, and then about 40 minutes worth of behind-the-scenes, making of, and source material featurettes. Very good for what was included. It also looks great on blu-ray.
It does have some flaws, but overall, they are, in my opinion, minor in the larger overall context. It is definitely worth checking out.