Ok, let's get the worst thing out of the way first: the relationship between Michael (Scott Speedman) and Selene (Kate Beckinsale) is extremely weak. I think I understand some of the underlying motivations for the two, but this is one very important aspect of the movie that really needed to be better fleshed out. How bad is it? It's on the scale of Anakin and Padme in AOTC. However...*SPOILER AHEAD*
The good news is, there is quite a deep backstory to the whole movie, with a more-complicated-than-your-average-bear plot. I would say that it's got a deeper backstory than say, Blade, which I absolutely love. There's enough there that I'd like to learn more about the history of the Vampires and Lycans--a good thing in my book. For some, it will seem like the movie falls short with its machinations, twists and turns. Some will think that the twists are pretty obvious, but I think if you just go with the ride that the story takes you on, it'll be pretty enjoyable. If you're sitting there, constantly trying to outsmart and outthink where the movie is going, then you're going to be disappointed. (Hey, nothing wrong with that if that's what you want to do--I've done the same with many a movie!)
As for some of the issues that have been raised in this thread, I've got some thoughts to add.
GUNS--There are a lot of guns and gunfire in this movie. If the idea of Vampires and Werewolves using guns to fight each other turns you off, RUN from this movie. If, however, you accept the idea that, like humans, Vampires and Werewolves find the firepower of guns far more efficient for killing each other than claw-to-fang combat, you'll have a blast.
"MYTHOLOGY"--I use "mythology" because I don't subscribe to the idea of a unified theory of vampires and werewolves. The "mythology" that we've grown used to, e.g. sunlight kills vampires, silver bullets kill werewolves, etc. have always been tinkered with, altered and discarded. Let's face it: there's no canonical record of the history, biology, or physiology of vampires and werewolves. Bram Stoker himself excercised creative license when he appropriated folk beliefs in vampires when he wrote "Dracula". I think we all have our favorite little bits of "mythology" when it comes to werewolves and, in particular, vampires. But I think we need to be prepared to see some of these come and go if we're to continue getting movies involving these creatures. Besides, I think part of the fun of these movies is seeing what the film makers decided to go with (or not). I won't say what Underworld does with the "mythology", but there are elements which are retained, some that are discarded, and a whole bunch which are simply not referred to or dealt with.
Personally, I hope this movie does well because I'd like to see more of the story. Hopefully, some of you guys will go see it--I think it'd be fun to discuss some aspects of the story.