I do believe in UFO's... people who I know that aren't crazy have told me some pretty interesting stories (to say the least) but also take this into consideration...
Going by our current understanding of the laws of physics, and understanding of the universe, the universe is probably teeming with alien life but the problem is that a consequence of our current understanding of the laws of physics, space travel as we currently imagine it (in sci-fi etc) is pretty much out of the question.
To travel between stars we need to let go of the concept of time, because we can get to distant places within a human's lifetime, but the price you pay is that on Earth hundreds of thousands of years may pass while these astronauts travel to their destination, everything that they know of may be gone (including the human race).
Add to this the problem of an aging universe that is slowly dissipating into cold nothingness (again to the our best current knowledge), our ability to see distant objects is very limited, and the fact that our species has been around for quite literally the blink of an eye, and we can assume other species are also only around for the blink of an eye, so with the wealth of variables us running into an alien species is extremely unlikely.
I think that our understanding of existence is going to undergo a huge upheaval, and that's why I do skeptically believe its possible to have alien visitors, because an existence changing discovery is just around the corner and it can't come soon enough.
PS. Sorry for the lecture but I really really love astro physics so I can bang on and on forever about this stuff...