The universe is a very big place, and I certainly believe that there is alien life out there somewhere... probably in lots of places.
Whether any of them ever stopped by our own neighbourhood is a different question entirely. I doubt it. If they did, the odds are that it happened during the Precambrian, since that Era makes up about seven-eights of Earth's history. So there wouldn't have been anyone around to greet them except bacteria and maybe a few worm-like creatures. Not much of a welcoming committee.
And UFO's?... I have my own hypothesis about them.
My UFO hypothesis is no more crazy (and also no less crazy, be warned!) than the usual flying-saucer-with-little-green-men interpretation. It explains most of the common attributes of UFOs, including the shape, color, erratic behavior (hovering, then zooming away), electrical/magnetic effects, invisibiity to radar...
Does anyone want to hear it, or is this the wrong thread for that?