I'll believe it when I see it. Maybe.
Don't get me wrong: I totally believe there is probably intelligent life "out there". And I hope they have traveled here and that we have found signs. But based on what we know (or think we know) about the laws of the physical world we live in, it doesn't seem likely that craft from other worlds have visited us.
Maybe if you consider that they might be unmanned probes it starts to become a little more plausible. But crewed ships, and especially ones that are meant for a round trip? Unless there are habitable planets a LOT closer than we think, or the aliens have made some amazing scientific/technical breakthroughs, it's hard to imagine. Maybe if the aliens are really long lived and tolerate boredom incredibly well... or have invented "suspended animation"?
