TLC (or a channel along the lines of that) ran a special on the "Phoenix Lights," and I'm 100% sure they're military flares. Two reasons:
First of all, if you have ever seen flares, they look pretty much identical to the alleged UFO's. But lets face it, that's hardly enough concrete evidence... However, that leads me to my second post: in the infamous Phoenix Lights videos, there is a mountain range beneath the lights. The lights eventually appear to "shut off," essentially rendering them invisible. During the special, they go to some computer visual specialists and they superimpose the mountain range onto the video... the lights disappear almost
exactly in physical conjunction with the mountains. It's hard to explain, but, basically... if flares were falling towards the ground, you'd obviously lose sight of them as they fell behind mountains, provided the mountains were physically between you and the flares (which, based on research, they appear to be).
OK, now what about my personal opinion on the topic as a whole? Well, it's really tough for me to say.
On one hand,
Yes, I do believe aliens exist. In fact, I would almost guarantee it. But just because I believe that does not mean that I believe they fly around in spaceships here on Earth. In fact, I find that incredibly unlikely. Look at how common place sightings are. It's been like 60-70 years now since this buzz has started. Why is that that we still don't have anything close to resembling some concrete evidence!? I really hate the "government coverup" crap. I just despise hearing that. That is such a weak argument for people to use. Its a crutch that's also available to throw in there at any time.
Back on track though... of all the videos and photos that I've ever seen, I have yet to see anything with some detail or distinction. The ones that aren't obvious hoaxes are usually just blurry lights thousands of feet away. I hardly consider that "evidence." You'd think that if they were here, and were visiting as frequently as is claimed, we would snapped some up-close footage that was nearly impossible to deny. Unfortunately, that's not the case...
But then, again, part of me does believe. There is a lot of eerie stuff floating around there. Like this video, for example:
http://www.youtube.com/watch?v=RcrzD0PfyPs
I find that just chilling!
If I had to pick one way or the other, though, I'd say that extraterrestrial beings have not been here. It just seems too unlikely to me. I think ancient pictures and writings can best be explained as either ignorance (weather/space phenomena) or with metaphors (eg: we'd not supposed to interpret what they drew/wrote literally). While there are a lot of military UFO sightings, I'd say that those can be best explained by enemy military spy planes, and things of that nature..
If you believe in UFO's because you've examined the entire body of evidence objectively and have reached that conclusion... fine... but I think there are very too many people out there who force themselves into believing (placebo effect, I guess) due to the fact that an alien-filled world is much more interesting and romantic than one that isn't.