I am a scientist and philosopher. With that background in mind, let us examine some of the claims made here.
I'd like to begin by saying that it is the primary goal of science to describe/explain what we observe, and secondarily, to allow us to observe things we couldn't observe before. That said, science will necessarily eventually (or maybe it already has?) explain the nature of UFOs. As weird as the odds seem that a 100 people could simeltaneously hallucinate a similar or same object, it's actually far, far, far less likely that it's aliens. Note that this argument has no bearing on whether or not there are aliens in the universe. That is irrelevant to the discussion here. If we can't all agree that there probably is other life out there, then we can almost definitely agree that we are probably not ever going to be able to prove there isn't. So we should accept the possibility of life elsewhere as a valid premise.
I'll start with the most recent, the temporal traveler. Logically, if we allow for backwards time travel, and allow for an infinite future (a necessary thing if you have time travel. You needn't ever reach that end), then all moments everywhere in the entire universe will likely be filled with time travelers. It's a fascinating idea, but the logical problems are common in it, so I don't think it's a worthwhile place to investigate at the moment. I can think of quite a few right now. Nonetheless, an excellent thought experiment.
And now I'm forced to debate probabilities of spontaneous life as we know it. I won't bother with it too much, except that, within the scope of our universe as we know it, it is far, far, far more likely that there is an advanced species of civilization on earth that we don't know about than there is to be one elsewhere.
In regards to spatial distortion as a means of instantaneous travel, it's an interesting idea. However, we note that our UFOs appear to move very quickly. This is a problem, because UFOs should never move at all if we accept the hypothesis of some sort of far reaching spatial distortion movement. They should merely exist where they're wanted.
One of the most telling problems is the aliens' apparent inability to disguise themselves...as mentioned earlier, they apparently hovered in front of a school? This is very atypical behavior for a species that wishes to remain secret. I assume that they do, otherwise, they would just hang out with us face to face. This is of course, from a human culturocentric perspective, I suppose.
We're also attributing far too much personality to the aliens. I can't think of much of a good reason at all for human beings to have free will, conscience thought, etc. We should be open to the idea that our visitors resemble humanity in no way whatsoever...and are more comparable to ants that can use tools. From a Dawkins genetic perspective, and we assume that the premise is true of any life anywhere (although the actual mechanisms will be quite different), their bodies and "minds" are merely tools to help the genes replicate.
What's with the dissimilarity between the UFO vehicles? I'd imagine there'd at least be a consistent theme, a most efficient form. Cars or aeroplanes all basically resemble each other, but not so among UFOs.
It's important for scientests to challenge ideas and shoot them down whenever possible. Once an idea is cannonized, we build new theories based on it. Therefore, it is extremely vital to avoid building false premises.
Something is happening, and that's about the extent of the real world knowledge. If there was good proof, we wouldn't be having this debate.
I actually have seen a UFO in real life once, although I don't attribute to aliens (I personally don't believe there are aliens on or near earth). It was bizarre, like nothing I've read about. It was night on a country back road, late at night, probably 2 am or so. It was hovering silently, not moving at all, about 100 feet off the ground...not very high. In size and vaguely in shape it resembled a helicoptor. It had giant spot lights all over it, maybe as many as six off the top of my head. It didn't really appear to have any magical properties, the light, it looked like yellow artificial light, as we actually do get out of our spot lights. The giant beams of light appeared to be pointed in random directions...they were at weird angles to the craft. I had two friends in my car and both saw it.