And then they take their masks off, and reveal they're some sort of horrible lizards ? Sorry- I think I just revealed how old I am...

To answer the original OP, I can believe that the presence of intelligent aliens is possible. The only reason that such a species would visit our planet without either (a) meet with us and unlock secrets of the Universe, or (b) horribly conquer/enslave/torture/genocide the human race to steal our planet and/or its resources, is that they intend to study us, for some unknown purpose.

A basic tenet of a scientific study under controlled conditions is that the test subjects must act as they would normally. This would certainly preclude knowledge that a sinister study was being imposed upon us. That said, why would aliens allow the test subject (invariably an unemployed, drunk man from Arkansas, or else a middle-aged spinster with more than 9 cats) to be anally probed, and then return without a complete memory wipe ? Unless nearly 100% of all self-proclaimed abductees are delusional, which is possible...

Spying on us wouldn't really be that difficult for an advanced space-faring race. One wonders why they would even need to interact with us on such a wide scale. We ourselves, do a pretty good job of spying on ourselves, and the pinnacle of the "dominant" civilization right now seems to be iPods and NASCAR. An alien species could just take refuge on a nearby planet, or assume orbit inside of Jupiter, and just monitor transmissions. Maybe every 10 years, take a junket to Earth and steal a couple of dozen humans, sheep, donkeys, and bamboo plants. Simple, effective- and no need to alert everyone on the ground with blinking lights in the sky.

It's more likely that UFO's are our own creation, like the so-many Russian military test aircraft from the 50's to the 80's spotted in the countryside of the Steppes. Or else they are things that most of us don't understand, like a decaying satellite, meteor shower, or "pocket of swamp gas reflected through a weather balloon...".