Some of the theoretical arguments against the existence of other life much less other intelligent life, or if it actually exists its ability to get here, seem persuasive.
But real data, evidence, always trumps theory in my opinion. These cases were real events occurring to real people that presented as described. The testimony and other evidence can't reasonably be dismissed just because they appear fantastic. Especially with good observers like pilots and police officers whose testimony would otherwise be accepted in a court of law. The burden is on the skeptic to credibly demonstrate how these are actually misperceptions, hallucinations, errors, hoaxes, useless anecdotes, etc.
In the absence of that the reasonable conclusion is that some people have observed strange apparently structured material objects in the atmosphere that give the strong impression of being vehicles.
The best cases stand on their own merits as evidence that on some rare occasions what seem to be alien vehicles appear to humans, sometimes producing physical effects including radar returns, radio interference, ground traces and leaving images preserved on photographic film or electronic media. In my opinion the extraterrestrial hypothesis remains plausible as the explanation.
Just a sampling of the best data:
The 1947 Kenneth Arnold sighting
Except for the WWII "foo fighters", this begins the modern era of UFOs. A good analysis is at
http://www.martinshough.com/aerialphenomena/Arnold%20analysis2.pdf . There do not seem to be any valid optical, geometric, geographical, psychological or other reasons to doubt the major features of Arnold's sighting as reported and they are internally consistent. The analysis results in a range of 16-20 miles, a minimum length of 70-90 feet, and a speed of 890 to 1200 mph. Arnold described the objects as trimmed-off in the rear thin shiny "saucer-like" discoids reflecting sunlight blindingly like metal at certain angles.
http://www.ufoevidence.org/cases/case116.htm The Chiles-Whitted Case - Montgomery, Alabama, United States - July 24, 1948 - UFO Evidence
http://www.ufoevidence.org/cases/case92.htm The Nash-Fortenberry Sighting (aircraft encounter with formation of UFOs) - Virginia, United States - July 14, 1952 - UFO Evidence
http://www.ufoevidence.org/cases/case665.htm The RB-47 UFO Encounter - Gulf Coast Area, United States - July 17, 1957 - UFO Evidence
http://www.ufoevidence.org/cases/case90.htm Socorro / Zamora UFO Incident - Socorro, New Mexico, United States - April 24, 1964 - UFO Evidence
http://www.ufoevidence.org/cases/case104.htm Coyne Helicopter Incident - Mansfield, Ohio, United States - October 18, 1973 - UFO Evidence
http://www.ufoevidence.org/cases/case86.htm The Cash-Landrum Case - Huffman, Texas, United States - December 29, 1980 - UFO Evidence
http://www.ufoevidence.org/cases/case287.htm Japan Air Lines Flight 1628 Over Alaska - Alaska, United States - November 17, 1986 - UFO Evidence
http://www.ufoevidence.org/cases/case1125.htm Belgium Triangle UFO Sightings - Belgium - October, 1989 - UFO Evidence
http://www.ufoevidence.org/cases/case277.htm Illinois Triangle UFO Sighting (by multiple police officers) - Illinois, United States - January 5, 2000 - UFO Evidence
UFO studies
The 1999 French Cometa committee report, summarized at
http://www.cufos.org/cometa.html . An in-depth study of UFOs, covering many aspects of the subject, especially questions of national defense. The study was done over several years by an independent group at the Institute of Advanced Studies for National Defense, or IHEDN, and by other qualified experts from various fields.
http://www.nicap.org/papers/92apsiee.htm Fifty-Six Aircraft Pilot Sightings Involving E-M Effects - Haines (1992)
Abstract: "Reports of anomalous aerial objects (AAO) appearing in the atmosphere continue to be made by pilots of almost every airline and air force of the world in addition to private and experimental test pilots. This paper presents a review of 56 reports of AAO in which electromagnetic effects (E-M) take place on-board the aircraft when the phenomenon is located nearby but not before it appeared or after it had departed. These effects are not related to the altitude or airspeed of the aircraft. The average duration of these sightings was 17.5 minutes in the 37 cases in which duration was noted. There were between one and 40 eye witnesses (average = 2.71) on the aircraft. Reported E-M effects included radio interference or total failure, radar contact with and without simultaneous visual contact, magnetic and/or gyro-compass deviations, automatic direction finder failure or interference, engine stopping or interruption, dimming cabin lights, transponder failure, and military aircraft weapon system failure. There appears to be a reduction of the E-M energy effect with the square of increasing distance to the AAO. These events and their relationships are discussed. This area of research should be concentrated on by other investigators because of the wealth of information it yields and the physical nature of AAO including wavelength/frequency and power density emissions".
http://www.ufoevidence.org/documents/doc622.htm Interference with Aircraft Equipment (Sturrock Panel Report) - UFO Evidence
Summary: Richard Haines presented a summary of his extensive research into pilot-UFO-sighting reports. He now has a catalog of over 3,000 pilot reports, of which approximately 4% involve transient electromagnetic effects associated with the presence of strange objects.