oh, undoubtedly, i don't think we've ever been "visited" and i don't think we will be any time soon, but i'd love to be wrong.
I have the opposite opinion from what I've read on the subject.
To me the best UFO cases are those reported by military and air traffic sources (specially pilots) since they are made not by the general public but by specialists and in many cases are backed with data (specially unidentified radar tracks). These cases are a minority when compared to the rest of the UFO reports but usually the incidents are usually classified as 'reliable' (several witnesses, images/radar records, etc.) and 'unexplainable'.
And, usually, the official government documents were originally classified as either 'secret' or 'top secret' before they were released to the public. And while the official position is that the phenomena doesn't pose a defense threat, some of them kinda make your neck hair stand up. Just do a search for the Malmstorm Air For Base Nuclear Missile UFO incident, where Air Force personnel reported that a hovering UFO simply hovered over a missile silo and deactivated 10 nuclear missiles inside. And that this happened again on the same day at another nuclear silo on the same base. Or, in another example, the military organization responsible for tracking the skies/space over North America, NORAD, has released documents stating that it had detected over 7,000 unidentified events over a 10 year period in the 1970s. Let's say that 99% of those can be explained as natural phenomena... but what about the remaining 1%?
Now, I understand the official government position of dismissing or refusing to provide explanation for those cases, while at the same time there are official instructions on how to report those incidents, because I honestly don't think they have better options. If they say that aliens are involved, they would need to produce proof. And even if they had proof and revealed it, they'd be admitting that they don't hold complete power anymore.
And also, because I don't believe that whomever is behind those phenomena wants to reveal itself. If they wanted to let the world know about their existence, they would have done it so by now. And while aliens are the usual culprits, there are other possibilities as valid, like time travel, other dimensions, or even humans from Earth. But to me, there's something clearly out there and examining us, since so many valid (IMO) reports exist of actual unidentified craft engaged in deliberate behavior.