If you liked the previous underworld movies I am fairly certain you will enjoy this one as well. There is lots of Vampire against Werewolf fights and the background story for the other two movies is explained. Without Kate Beckinsale I was a bit unsure if this would feel like a Underworld movie, but thankfully I was proved wrong.
The story is solid enough and really clears up a lot of the background. We also get to see the Lycans in a very different light. In this movie they are the good guys in a way, and the vampires, with Viktor in the lead, are the bad guys. Of course this line is gray and we can discuss if Vampires and Werewolves are good guys at all. It is worth watching this to get a fresh aspect of the whole fight between the races and to fully understand what is going on in the later movies.
Another aspect I really liked about this film was the great performances given by Bill Nighy playing Viktor and Michael Sheen playing Lucian. Without them this would have been a dismal affair. That's not saying the rest of the cast was horrible, but they did outshine the rest.
Sadly the movies has some flaws as well. The dialog seems very stiff and unnatural. I have a hard time believing Vampires talk the way they do in this movie. You can also see that the special effects budget was lacking. Especially the blood looks fake. These are however minor flaws and I was positively surprised by the movie. It keeps the feel of the previous films and adds new light to the fantasy world of Underworld. 7/10