From a scientific standpoint, I neither believe nor disbelieve. I have simply not seen any proof of extraterrestrial life. We can go on and on about how big the universe is and therefore how "likely" it is that there is extraterrestrial life out there, but we've barely begun exploring our own solar system, never mind any of the rest of them, so we really don't have a big enough observational sample to say whether life-conducive conditions are as common as dust in the universe, or if our planet is an entirely unique "snowflake," and life is an anomaly peculiar to this one place.
And Dave Todd, I must point out that I'm a Catholic, so I do believe in God, and doing so does not cloud my views on the matter either. I'm not afraid that contact with an intelligent alien race will destroy the foundation of my faith, because there's nothing in my faith that says that we're the only intelligent species.....or that we're not the only intelligent species. It remains to be seen.
If there are technologically advanced alien species out there observing us, I hope they are benevolent. In our own human history, when technologically advanced cultures encountered less technologically advanced cultures, things usually didn't turn out very well for the less technologically advanced cultures. Technological advancement doesn't require sophisticated morals or ethics, just enough sense of self-preservation not to trigger an extinction-level event, like full-scale nuclear war.