Mostly, I've heard that 9/11 was primarily a failure of intelligence. However, the question is: how much can one reasonably expect from an intelligence agency?
One of the reasons the 9/11 attacks went off relatively well for the attackers was that their plans required relatively few people to know what was really going on until it was too late. Basically, there were the people who came up with the plan; the people who arranged for the "team" to be recruited; the and the "team" itself. No one else necessarily needed to be privy to what they were doing. No one else presumably even noticed suspicious activity, if the planning and preparation was done right - which it was. The hijackers even learned how to fly planes here in the United States.
The fewer people know a secret the less chance someone will blab.
This is one of the key reasons that the official story is more plausible than most (if not all) of the conspiracy theories out there. As demonstrated above, in order for the official story to be true, relatively few people needed to be in on the thing. In order for the US government to have been complicit in the attacks, a lot more people would have to have been in on the whole thing, depending of course on the level of involvement you purport our government to have undertaken. If you are asserting that the government knew and deliberately did nothing, than far fewer people needed to know than if you are purporting, first instance, the "Controlled Demolition" theory, which just about debunks itself when one considers how many people would have to have been involved and who would have to have been silenced about their involvement in the wake of 9/11.