So what are they? These things are physical objects that have performances that are unlike anything we can produce at this point. No wings, no perceivable jet or rocket engines. The ability to move at speeds in our atmosphere and to make course corrections that defy all known laws of physics. The only real "sensible" answer would be secret black project military vehicles that have been encountered for decades. Foo fighters were seen by allied & axis air crews throughout the second world war. These observable craft could perform like no aircraft then or now. Were they part of some secret weapons system developed during the 30s/40s? Unlikely. So what are these things and where are they coming from? The evidence is there and now officially released by the American military. Finally the US has admitted that they have been spending millions of dollars on research into these objects, 50 years after the very public closure of project blue book. I ask you...Would the United States with its massive scientific infrastructure be spending money on research for something that simply doesn't exist?
OK, so what have we got?
On the one hand:
A species of beings which evolved intelligence and developed a high-tech civilization. They are not from our solar system (we can be sure of that), so they must be from another star, the nearest of which is 4 lightyears away. That means either an FTL drive or the ability to endure many years of STL travel. They will know that the Earth probably has intelligent life (we could detect ourselves easily at 4 lightyears), but their motive for coming here is utterly mysterious (alien?). If they wanted to invade (why?) they must (a) have studied us enough to know how to conquer us; and (b) have huge resources standing by (where?). What are they waiting for? If it's just exploration and research, which is a reasonable idea, why are they so bad at concealment? And, as someone else has pointed out, why so many different manifestations and apparent ship models? And why are these ships clearly identifiable as such? Also, the claims that they've been around for decades, if not centuries, don't make sense either. What the hell have they been doing all this time? They don't seem to have done much except hang around and be mysterious. Why have we not detected their civilization? If we could detect ourselves at 4ly, then we could detect them.
On the other hand:
A series of observations made up of individual claims (why has there not been a single unequivocal sighting?). People interpret what they think they see in many ways, so we have to rule out the possibilities of optical illusions, malfunctioning instruments, misjudgments of speed and distance, sheer mendacity, publicity-seeking, experimental aircraft of unfamiliar design. Why would governments want to hide the findings anyway? Why did sightings peak during the early Cold War (just as SF writing was at its peak, experimental aircraft were everywhere, some technology, especially jets, was still fairly new to many people, and paranoia was a way of life)?
Now I know that Occam's Razor is a two-edged sword: each side believes their ideas are the 'simplest', but I'll take a B58 at mach 2 over an almost-invisible FTL ship any time.