We've all seen it happen in science fiction films. The expendable character is outside the space ship, or trapped in the air-lock. The space suit is damaged, or the helmet is removed. His friends frantically bang on the glass as we get a nice close-up of his face as it blows up like a balloon. And then... well it depends on the film. Sometimes he freezes solid and shatters like glass, other times he explodes in a messy shower of frozen flesh. Is this realistic?
The one film I can think of which got it right is 2001: A Space Odyssey
when Dave Bowman has to get back into Discovery after being locked out by HAL9000. We see him take a deep breath in the pod's airlock, open the door, and get expelled into Discovery's pod bay. There's much frantic bashing about as he bounces off the walls and claws himself to the manual override switch so that he can close the bay and restore atmospheric pressure. The entire scene is filmed in absolute silence, until the air is turned on. He's exhausted by his ordeal, but otherwise unharmed. The only problem here is that he should not have tried to hold his breath - he would probably have injured his lungs trying to hold in the air against the vacuum.
But shouldn't there be some dramatic effects? After all, we know from very simple experiments that water boils at room temperature in a vacuum, and we've all seen balloons and other objects explode when the pressure outside is too far below the pressure inside. Well there's two ways to deal with this. First, we can discuss the theory, as explained in this excellent piece extracted from a NASA article:
If you don't try to hold your breath, exposure to space for half a minute or so is unlikely to produce permanent injury. Holding your breath is likely to damage your lungs, something scuba divers have to watch out for when ascending, and you'll have eardrum trouble if your Eustachian tubes are badly plugged up, but theory predicts -- and animal experiments confirm -- that otherwise, exposure to vacuum causes no immediate injury. You do not explode. Your blood does not boil. You do not freeze. You do not instantly lose consciousness.
Various minor problems (sunburn, possibly "the bends", certainly some [mild, reversible, painless] swelling of skin and underlying tissue) start after ten seconds or so. At some point you lose consciousness from lack of oxygen. Injuries accumulate. After perhaps one or two minutes, you're dying. The limits are not really known.
You do not explode and your blood does not boil because of the containing effect of your skin and circulatory system. You do not instantly freeze because, although the space environment is typically very cold, heat does not transfer away from a body quickly. Loss of consciousness occurs only after the body has depleted the supply of oxygen in the blood. If your skin is exposed to direct sunlight without any protection from its intense ultraviolet radiation, you can get a very bad sunburn
Alternatively, we can look at the case in 1965 when Jim le Blanc was trying out a space suit in a vacuum chamber. The suit developed a leak, and lost pressure. He remained conscious for 14 seconds before passing out, which is about the time it takes for blood to move from the lungs to the brain; his brain was suddenly cut off from oxygen. His last conscious sensation was of the saliva boiling on his tongue and it seems safe to assume that his tears would have bubbled away from his eyes as well. The test team immediately opened the valves and it took about 15 seconds for air pressure to be restored. Jim recovered fully before the team doctor was able to reach him. Here's a brief video about the event, based on documentary footage: http://www.youtube.com/watch?v=KO8L9tKR4CY&feature=player_embedded
The video does get one thing wrong though, when it claims that no astronaut has ever suffered catastrophic de-pressurisation. In 1979 the soviet Soyuz 11 mission, on its way home from the first ever successful docking with a space station, suffered de-pressurisation while preparing for re-entry. All three crew-members were killed. So while the effects of exposure to vacuum are nowhere near as dramatic and exciting as portrayed in popular film, they remain very serious and dangerous.