I really would like to believe we have been visited by alien life forms, but the more I watch "The Universe" on the Discovery Science channel, the less likely it appears. The universe is so incredibly VAST, a trip to the nearest star is beyond any known method of transport, even at light speed which is the universal speed limit according to Einstein. Of coarse, that is barring theoretical methods of travel (i.e. wormholes, warp speed, parallel universes, time travel, etc) which I can't include. IMHO, God created the universe so vast as to prevent intelligent life forms from ever meeting and contaminating each other with their customs, prejudices, beliefs, etc, but that's my opinion.
Having said that, with all due respect to those who have "seen" these vessels, the numbers of stars and planetary solar systems that may contain planets like earth in the Milky Way alone are estimated in the billions. So while I doubt VERY seriously the existence of visitors to our planet, the numbers alone can't rule out the existence of intelligent life elsewhere in the galaxy/universe. In fact they very much support the possibility. So until I SEE irrefutable proof they exist, I'll have to rely on Hollywood to satisfy my curiosity about UFOs.
Just my opinion.