I think you may be wording your question wrong. UFOs are 100% real, as a fact.
An Unidentified Flying Object is an "unusual apparent anomaly in the sky that is not readily identifiable to the observer as any known object". Some things that can be called a "UFO" could be a plane, kite, weather balloon, etc. If it can't be identified (due to speed, weather, or light conditions), then it is a UFO. It could be as common as a bird or a tumbleweed kicked way up into the sky by wind. UFOs happen, but they're not all little green aliens.
To answer your question, do I believe in extraterrestrial life that is flying some type of UFO near planet Earth? I'm not sure about that. I doubt it. But anything is possible. So "yes" to the possibility.
...however, my girlfriend and I did see the Marfa Lights together, one night driving through West Texas (moving her from San Fran back to Austin), we stopped at the viewing platform around 3am. Though I am sure there is a scientific explanation (currently there are several theories, no one has proved it yet), it sure was weird. Floating orbs dancing in the pitch black distance, lazily floating 100 feet over the horizon, then rapidly jumping a distance of .5-1 mile within a fraction of a second. All in absolute silence with zero light pollution. Strange stuff. I figure it was a lot like the Aurora Borealis. Magical to view, but all science.
Photo below doesn't do this phenomenon any justice...