I know what you're thinking: Why is this one in both the Bad and the Good sections? While the movie did blow it--literally--at the very end, there was a lot of good science in it.
The comet is discovered by an amateur, which until recently was how most comets were discovered. When the astronauts go to the comet, they can't land on it due to its weak gravity, so instead they tether themselves to it (I literally cheered when I saw that scene in the theater). And when a mile-wide comet fragment plummets to Earth and creates a 600-mph tsunami, the scene was very accurate, except for only a few small details.
This fictional display of the raw, destructive power of gigatons of rock and ice slamming into Earth was a wake-up call. We do need to be concerned about cosmic debris.