I've only ever seen the first Underworld, and I probably would have avoided this one except tickets were free. But, it seems that the most important elements of the film were covered in a brief flashback sequence in the first film (although with a different actress).
I'll be honest, I fell asleep during Rise of the Lycans, only awoken by werewolves (lycans, whatever) and vampires tearing each other new assholes.
Unfortunately, no matter how good CGI gets, werewolves continue to look weird and kind of lame.
By the way, contrary to what you might think, Kate Beckinsale is not in this film (not that it matters), the girl in all the previews just really looks like her.
On a final note, what is Michael Sheen still doing acting in movies like this? After playing Tony Blair in The Queen, the late Portland State alumn Art Honeyman in Music Within, and David Frost in the Oscar nominated Frost/Nixon, why is he still playing Lucian the Lycan?
I give this movie 1.5 stars for brevity, and only waking me up occasionally while I napped.