Culturally, I don't think it's possible to overstate the US influence on the rest of the world. Whether it's music, film, TV, fashion, etc., we have a huge influence on the rest of the world.
Economically, I think the US has declined slightly since 9/11. We've spent so much of our own money chasing the boogeyman that we've neglected investment in our own country. That's going to haunt us for years.
Politically, I the US has declined a lot since 9/11. Our aggressive attempts to make the middle east like us has made us look bad. Our unilateral use of force has mad us look bad. Our use of torture has made us look bad. Neglecting our own people (healthcare, unemployment, immigration, etc.) to further our pursuit of terrorists has made us look bad. I had hopes that the talks with Iran and getting Syria to destroy its chemical weapons would put us back on the right track, but it appears that Russia is going to make that very difficult with what it's doing in Ukraine. We have a long way to go to get back to where we were in the 90's when it comes to our standing in the world politically.