Deep Impact is most often compared to its death-comet partner from the summer of '98, Armageddon. Deep Impact is a drama; Armageddon is an action film, and delivers just what we would expect from an action film, namely, over-the-top characters, a simplistic storyline, and an abundance of special effects. Deep Impact presents just the opposite: Characters that are notably human, several dovetailed story lines, and it saves the special effects (which are very good) for the movie's climax.
Armageddon did better at the box office primarily because it was much more hyped, and because it featured an A-list star (Bruce Willis) while Deep Impact did not. Its enjoyability, though, is very limited: If you are not a fan of the action genre, you will not like Armageddon. Deep Impact is the substantially better film and reaches out to the viewer to a far deeper degree.
As you certainly know, the plot revolves around the fact that a seven-mile-wide comet is on a collision course with earth, and if it makes impact it will represent an Extinction Level Event (i.e., the death of all life on the planet). Having about a year and a half's notice of this, the U.S. and Russian governments send a spacecraft, the Messiah, to destroy the comet by drilling nuclear warheads into its core and then detonating. The movie focuses on three primary story lines: 1. The young reporter Jenny Lerner (Tea Leoni) and her struggles with her career and her parents; 2. The high-school couple of Leo Biederman (who discovered the comet; Elijah Wood) and Sarah (Leelee Sobieski); 3. The crew of the Messiah.
All three story lines are done in such a way that the viewer easily sympathizes with the very believable characters. The best done of the three is the spaceship's crew, although the most time is spent with Jenny. They all suffer from the film's only notable problem: The story lines seem somewhat rushed. Considering its broad scope, Deep Impact clearly would benefit from an extra 30 minutes to develop, especially with the underdone angle with Leo and Sarah, but the directors evidently decided two hours was all they could use.
Deep Impact, as I mentioned, lacks an A-list star, but it does feature superb performances from two of the best supporting actors of our generation: Robert Duvall (Spurgeon Tanner, captain of the spaceship) and Morgan Freeman (Tom Beck, the U.S. President). Duvall is definitely the standout of the film with an A+ performance as Tanner.
As for the other actors/actresses: Tea Leoni (playing Jenny Lerner) gets the most face time in the film and delivers a believably good performance. Maximillian Schell as Jenny's father is the one notable casting mistake; I'm not sure what they were going for with him, but they could have done better. Venessa Redgrave does well as Jenny's divorcée mother.
Elijah Wood (now a star but at the time just an up-and-comer) works very well as the teenage Leo Biederman, and Leelee Sobieski as his girlfriend Sarah gives us as good a performance as we can expect, considering how woefully underdeveloped her character is. The film arguably devotes a bit too much time to Jenny and her father and not enough to Leo and Sarah.
If you haven't seen this movie yet, it should be at the top of your must-see list. The film moves at a good pace (if a bit fast), grabs your attention at the beginning and holds it throughout, and it features a truly exceptional final 20-25 minutes. What stands out most about this movie is its human touch and sensitivity. It manages to probe an impressive array of human emotions in two hours' time, and it will leave you with plenty to think about -- although it probably will not leave you with dry eyes. There are precisely three movies I have seen that caused the room to get dusty around me (if you get my drift), and this is one of them.
In conclusion: See this movie.