Big fan of the franchise here. I think the film was undeservedly panned--I guess I would never have made a successful film critic. The film was gritty, intense, and had the same stylish darkness of the first two Underworld movies. I would have preferred a longer movie also, with a little more plot, and a little more exploration of the underlying Vampire and Lycan mythology. Plus it would have been nice to have seen Scott Speedman's shirtless torso again, but at least the latest movie left some hope that his character Michael might still be alive in the off chance there might be another sequel. In the meantime, Theo James is pretty nice to look at. And Kate Beckinsale is still sizzling hot--and I say that as a man not ordinarily attracted to women.
The bottom line is that if you like the genre and enjoyed any of the precursors, then you would probably enjoy this.