The terrorists utilized one of the greatest accomplishments of Western technology (airplanes/flight), turned them into effective WMDs, halted a major American industry for several days (airline industry), destroyed two of the tallest buildings in America, collectively known as the World TRADE Center. Current estimates say that 9/11 cost Americans 1.2 trillion jobs, not to mention the billions in physical damage to New York City and the Pentagon. Also, they timed the attacks so one of them was on live national news when it hit.

I would say it was all pretty effective regardless of whether they were able to plant explosives in the towers or not.