I believe that the vast majority of UFO reports can be explained by low flying air crafts or experimental military planes out in the desserts, I don't particularly believe that aliens are visiting each in flying objects at this current time. I think most if not all of UFO sighting can be explained logically.
Most UFO sightings aren't about aliens or extraterrestrials. They are simply misidentifying natural or pre-existing phenomena in the atmosphere or human craft that seem different or weird. A select few sightings can be seen as high-tech craft or vehicles that don't move in the same manner as human-made craft. It remains to be determined whether or not these things are top-secret human technology or other intelligent beings.
Yes, I believe Philip J. Kass's assertion that most UFO sightings are simply misidentification of natural phenomena, because if they were true, there would be more concrete evidence to back them up. If aliens really existed, they wouldn't fly within a mile of the earth's surface and then not land. People like to imagine things.
I do believe his assertion that most UFO sightings are a misidentification of natural phenomena; for example, sun devils often get confused for UFOs. The thing is is that I do believe that intelligent life has visited earth. If there's an advanced species within the milky way galaxy able to achieve star travel, they have been here.
I think this has been proven to be true in most cases that have been investigated in the past. Humans will always try to find an explanation for things they see, and if you see a natural phenomena that you have not seen before, many people will assume it is UFOs as the first explanation through ignorance.