Like Snakes on a Plane, it's a film that shows its ambitions in its title: it's a light hearted piece of fluff, not a serious work like Watchmen. Having said which, herself said afterwards that she enjoyed it more: it's certainly not got the level of violence of Watchmen.
It's not a great film: nobody will be raising that claim. But once it gets going, its sense of irony, with a certain post-modern take on vampire slaying ("You mean the garlic, crosses, stakes, that stuff? Oh yes, everyone knows that these days.") carries it off, and it does manage to be fairly funny. Paul McGann plays The Vicar with a straighter face than Anthony Head ever manages with Rupert Giles in Buffy, and it's the right choice.
The weakest feature of the film has to be the very premise: Lesbian Vampires. It's incredibly cheesy, and there's no way to run it without the lingering shots of half naked women getting all sensual with each other as well as with their victims.