OK so lets get straight down to it, this is a romance between vampires and humans, this of course will lead to the twilight comparison being drawn up somewhere, don't let this put you off though, where twilight fails (essentially everywhere it possibly can) vampire knight gets right.
Unlike with twilight all the characters are instantly relate-able, it has good amount of action and also features some interesting sub plots underneath the romance triangle, a character trying to fight becoming the very thing he hates, somewhat racial tension issues between the vampires and humans, uncertain back story of the female lead, the mysterious element to the lead vampires intentions, all interwoven really well to give the story depth and keep you coming back for more.
Another thing vampire knight does well is convey vampires in the light they should be shown as, bringing across the ferality of vampires, the fact that they are at heart savage beasts when around humans despite them trying to fight it, oh and they don't sparkle thank god.
So yeah ultimately a brilliant vampire romance with more than just a love triangle going on, great art style and good pacing, seriously recommend people give this a go!