Underworld is an odd mix of many positive and negative elements. Some love it, others hate it, and the vast majority of critics simply don't care.
The best aspect of the film by far is its well-imagined fictional world. In this story, vampires and werewolves (called "Lycans") have been in conflict for many centuries, and continue their war in today's world. They live as underground gangs, ever looking for new ways to destroy each other. The vampires, using modern technology, have found ways to artificially create human blood, thus removing their need to feed on human beings. Additionally, the Lycans--unlike traditional werewolves--have the ability to completely control their transformations.
The major difference between the two factions is that the Lycans have the advantage of brute strength, while the vampires must avoid sunlight and rely on weaponry in order to stand a chance. Another rather refreshing spark of originality is that Underworld's vampires and werewolves are not supernatural beings at all, nor are they intrinsically malevolent. Instead, their abilities derive from a much more science fiction-esque mutated virus.
Kate Beckinsale plays Selene, a vampire "Death-Dealer" (Lycan hunter). When she meets a young man that may be the key to a long-forgotten secret of both the Lycans and Vampires, she delves into forbidden history, revealing some rather shocking truths.
The undersaturated, gothic visual style of Underworld is striking. This style makes the world of Underworld feel somewhat special, despite its lackluster character and emotional beats.
A rather surprising thing about the film is that, for an R-rated film, it is extremely light on mature content. For a film about a super-powered bloody gang war, there isn't actually that much gore, save for a specific scene or two. Even more surprising is the fact that—despite the film being centered on a vampire Kate Beckinsale wearing a tight black leather outfit—there was absolutely no R-rated sexual content whatsoever. I literally sat there after finishing the movie and thought "where was all the sex and violence?"
One place that Underworld truly stands out, however, is in its obviously-well-thought-out universe and mythology. Everything from the design and function of the characters' unique weaponry to the intricate history of the vampires and lycans is dense with detail. In fact, it's so dense that it can almost be hard to follow at times. There is a lot of exposition in this movie. A lot.
The main problem with Underworld, however, isn't its plot, its characters, or its world. It's simply that there aren't that many strong emotional beats in the story. Considering that this is largely a story of loyalty and betrayal (with a side of love), there should be a lot more punch in this movie's emotion. There's any number of reasons this could be: the flat and boring music, the only moderately-well-written script, the monotone feel of the directing, etc. In the end, this lack of heart is what turns Underworld from a good or great movie into a merely decent one.
Truth be told, the concept of Underworld is really its most enjoyable aspect. The mere idea of a "sci-fi vampire/werewolf gang war" makes this film at least passable. Kate Beckinsale perfectly fits—and defines—the character of Selene, adding some much-needed depth and interest to the otherwise bleak story.
If you're a big fan of gothic fantasy, I'd recommend this film. It's got some great ideas and a rather complex mythology that may hold your interest.