So many people around the world have reported alien encounters that I think it is more likely than not that aliens do exist and have visited Earth. There are many strange things in the world that have not been explained despite our best efforts. Alien visitation would definitely explain some of these things. We may find other explanations someday, but it is definitely a possibility that aliens have been here and will return.
Math experts will tell you that numbers never lie. Statistically, given the number of potentially habitable M-Class planets in just our arm of the solar system, it is arrogant and presumptuous of us to think we are alone. Who is to say that our form of life is the only intelligent life in the universe?
The universe is made up of millions of star and planets. it is very hard to believe that over million of years we have been the only planet to achieve life. As NASA has exhibited in its extensive research of other planets, there are know planets in our galaxy that at one point had water. Water is what creates life, so yes Ido believe we are not alone.
While I wouldn't rule aliens out, there so far seems to be little sign of their existence outside of science fiction. I do not believe that peace-loving aliens have visited earth, and it seems unlikely that something like that will happen during our lifetimes. It's very interesting to think about, though!