There are two ways of looking at Underworld: either as a mindless, yet stylised action film that happens to have vampires and werewolves as the protagonists, or as overblown convoluted tripe (with fights).
I tend to side with the former. Enough action to stop thinking is good sometimes.
The acting in the film is, however, appalling, as is the character interactions. Very formulaic stuff. Just don’t go expecting a work of genius and you’ll be ok.