There are two different meanings for the term U.F.O. In one, which is what it really stands for, Unidentified Flying Object, of course this is real. You see something you cannot identity that is, by definition, a U.F.O. What this question is implying is the second, informal, definition for the term. A space craft from someplace other than earth with your preference of alien in it. That I’m not sure of. I would like to think that there is life out there, but I don’t think it is visiting us. I’m not terribly sure that the life out there would be anything beyond a mold or bacteria, let alone spare ferrying.
For sake of argument, If it could cross the incredibility vast distances between solar systems, I don’t think we would want to run into them. I fear that they would be like us, seeing a primitive people and a cheap workforce. Granted there would be no way to tell what they would do, because they are completely different from us and having no frame of reference to base my speculation on. It would be incredible to see, but also immensely terrifying. A creature so technology advanced to travel here could wipe us out in an instant.
The alien abduction phenomenon is rather interesting. Of course it is very difficult to prove that something didn’t happen. I think that a lot of the stories that comes out are people that are looking for a quick buck. If they really believe what they said happened to them or not, I cannot tell you. What I do know is that I do not believe them. If a species is so advanced that they can make it here, why play catch and release with the people of earth? Just keep them, no one would know where they went. We would be much like fish, completely clueless.
Of course, people claim that there is a huge cover up spanning decades. Do you really think that the U.S. government has it so together, that they can cover up something as huge as alien contact for over sixty years? Just think of how many people would have known about it, either directly or indirectly in that time span. Something that big just cannot be swept under the carpet. With modern technology, if a space ship comes anywhere close to Earth it will be photographed the entire time. If people thought more skeptically, without grasping onto fantastic ideas, I think a lot of the alien stories would go away.
In conclusion, I think the little gray, green, or what ever color men people claim abduct them to perform experiments on just let their imaginations get away from them.