For the first time in the series, the filmmakers have decided to leave out the rock that probably was the reason the films have been successful: Kate Beckinsale. I'm guessing this was a production risk/choice, but it seems to have paid off. Selene's absence from the story is necessary to tell the story of how the Lycans rose to power, how Viktor was driven insane (though we pretty much already know how), and, as off topic as it may be, how simply awesome Michael Sheen is.
Very rarely do the actors make an action movie good. Most action movies rely on explosions and (surprise!) action to enhance the quality and generally feature run of the mill copycat characters. Sheen's Lucian and Bill Nighy's Viktor will always stand out in the realm of action/fantasy characters to me, and it's easy to see why. Sheen's turn as Lucian is fierce, determined, and one of my favorite performances in an action movie I've seen in years. Despite playing a (mostly) villainous role in the first film, Lucian is the hero of this film, and it totally gives you a new spin on things. Sheen's brave and dedicated performance really makes the character work, while Bill Nighy is once again effectively creepy as Viktor. Rhona Mitra's presence as Sonja is welcome, though she doesn't carry the same charisma that Beckinsale does, making her an ill advised choice to be Selene's replacement (as she so obviously is).
The action in Underworld, though obviously restrained by a lower budget, is once again great. There's some nice dueling, the obvious gore and blood galore, and a nice Lord of the Rings style battle at the end. What it comes down to is that Underworld does nothing new, but does everything right in maintaining a steady river of entertainment. I was never bored, and I don't think you will be either.
The film does answer a lot of questions and provides some nice insight to the Lycans (which were my favorite parts about the first two films), though it suffers from the same fate as most prequels do...even a casual viewer of this series knows which characters will live, which will die, and how it will end. The film isn't really about the resolution, but rather the journey to the end. In fact, it's made in such a way that you can certainly watch the trilogy in chronological order.
When it comes down to it, I liked 'Lycans' a lot. It's a worthy installment in the series, and certainly not a disappointment to me. It's got good action, a wonderful lead performance from Michael Sheen, and should satisfy fans of the series and fans of mythological stories altogether.