The terrorists are winning on all fronts. The United States is stretched thin across several international conflicts and the native populations of most Middle Eastern countries are beginning to see us as over reactionary bullies who shoot first and ask questions later. They are succeeding by destroying our standing in the international community.
Though, Al Qaeda may have lost most of its thunder, they are still organized and intent on spreading terror wherever they are able. As long as groups like Al Qaeda are organized, terrorism will prosper and terrorist will succeed. Governments in the countries that house these groups must be determined to completely exterminate terrorism in order for their organizations to fail. With the current rate of the spread of Islam, this will never happen. They claim to be a peaceful people and most are, but the Quran (their "Bible") speaks of hate and annihilation of "infidels". That doesn't sound peaceful to me, in the least. While most Muslims are not radical, there exist many Muslims with radical enough beliefs to fly planes into two skyscrapers and kill over 3000 innocent Americans.
Of course terrorists are succeeding. The foundation of the United States is cracking under the weight of debt partially taken up for the war efforts in Afghanistan and Iraq. As we cause collateral damage and devastate the infrastructure of other nations in the Middle East, they will only become more anti-American, fueled by the thoughts of their dead friends and relatives at the hands of so called liberators.
The terrorists are only able to control areas which are already suffering a devastating political crisis. ISIS for example, took control of parts of Syria because the country was already in conflict with itself.
Yes the odd few terrorists manage to get through and cause massive loss of life such as 9/11, but we don't see them taking over. They are never going to take over richer and more progressed countries such as the US, most of Europe, Russia or China!
If they were succeeding, then at that point most people will start thinking they could potentially take over large powerful countries. This simply isn't true! You don't see anyone in modern world having this thought about terrorists who have limited technology and a medieval mindset.
We have gone over 11 years since the horrific terrorist attacks of September 11th. I would consider that to be a valid barometer of how much the terrorists have succeeded, which is to say that they are not succeeding at all. While we live in a more cautious and paranoid world, this does not mean the terrorists are winning.