Personally, I find some of the stories about what UFOs are capable of to be fairly strong evidence that they may be alien spacecraft. But I don't completely rule out the possibility that they are terrestrial aircraft. Take the triangular or boomerang-shaped UFOs reported in Hynek, Imbrogno, and Pratt's Night Siege. Reliable witnesses say the craft hovered silently or nearly silently and moved silently at very low altitude. This much could be explained if the craft were a lighter-than-air vehicle, but how would such a craft be propelled? No one reported propellers or exhaust, so one is forced to speculate that something like the ion drive (nmp.nasa.gov/ds1/tech/ionpropfaq.html) used for some spacecraft might be employed. At this point, of course, we have entered the realm of sheer speculation about hypothetical secret aircraft.
Occam's Razor is a key principle to keep in mind here. Occam's Razor, of course, says that the simplest explanation of a phenomenon is probably the best and closest to the truth. So which is simpler--that aliens have visited the Earth, or that a secret aircraft is roaming the night skies for no clear and discernible reason? Looks like a toss-up to me.