This is a question of massive proportions.
Saying NO might not be accepable in majority of population since MEDIA ( internet,newspapers,TV and radio ) made its case by posting various material on this subject thus people created image that every second strange thing in the sky is UFO... even when that "ufo" is airplane,aerial phenomena or something manmade.
By saying YES ( with a proof or firm evidence of such ) would change world fundamentally. I can only say it like that because it would affect so many things in our lives that people couldn't even start to comprehend such an answer.
Yes we know people have reported UFO practically everywhere on earth or skies...i have seen massive number of videos and footage of such things, well i like being in the flow with current information stream. And i came to conclusion that we don't have evidence a firm proof of ETI visiting our planet, we do have loads of sighting reports, which means something was going on or still is..this is irrefutable fact.
So it all comes down to a simple mental subject... belief.. and it will stay like that until offical contact is made and reported on global scale.
I can say from my perspective i've seen footage that is just out there...simply cannot be explained by human mind...UFOs, most of those which in my opinion were very real looked like some sort of science fiction craft. In some cases the craft deployed another smaller craft and main part flew away while smaller one did some sort of survey ( that is just speculation, i don't have a firm idea of what it was doing ). At one time there was event with some sort of a probe observed at sea, which was just hovering there while crew recoreded it, what it was doing i don't have a clue but entire thing looked very spooky...