I didn't care for Stormwatch, to be honest, and only found it at all interesting once he'd thoroughly eviscerated and remade it. (The Aliens coda to that story is a strangely fitting, if unintended, allegory--the Authority fertilized it, grew inside it, and killed and replaced it)
But the Authority, on the whole, I think is just a proper re-envisioning of the SH team concept in the present century without nostalgia and reference to the past. But it's a book only as good as its writer. The current one, for instance, I like. And I liked Ellis, but, I know this is blasphemy(and maybe it's really Quitely I liked), Millar's was more fun, and it was the first time I saw any depth in the characters starting to bubble up. (By design. Ellis has said that the Authority was done with minimal thought to characterization)