How does he rank as a writer? I mean, in AH you usually have to take what you can get, but how is he as a writer in general?
As others said, Stirling is a mixed bag as a writer. His pros are that his concepts and premises, while not always rigorous, are usually entertaining and creative; his world-building is similarly good. His writing style is easy to read and well embellished, though he has a tendency to repeat certain writing tics across characters and novels. (A common one: "Some random topic X" [pause while everyone looks at the speaking character] "Fuller description and context of X")
Stirling's cons are in his characters; they're usually either heroic types or clear villains with very few in between. More egregiously, the villains usually have very weakly developed motivations and are oftentimes just being evil for the sake of being evil (I want to conquer and rule this area JUST BECAUSE). Neither heroes nor villains display much character development, which can be sorta grating and Mary Sue-ish in the heroes. Classic example for this is Rudi, who throughout the Emberverse series is a perfect heroic figure and so handsome and noble yet humble and pages upon pages are devoted to
other characters having internal monologues musing on how perfect Rudi is.
All that said, Stirling is an entertaining writer and I owe a lot to him and Turtledove for sparking my interest in AH. I read anything of him that comes out (through the library) though I really wish he would wrap up the Emberverse and write a follow-up series with the Nantucketers...that was a lot more straightforward, and therefore enjoyable to me since I prefer tales of exploration and invention in an ISOT scenario as opposed to the Emberverse's mysticism and magic and all belief systems are real and we are just pawns of the gods.
Not to get too off-topic, but Isketerol (the guy who becomes king of Tartessos) and McAndrews, the guy who runs off to Egypt, provide some grayer villains/opponents of the Nantucketers. Walker and his clique are there to represent the worst parts of the 20th century.
Yeah, Isketerol is one of my favorite of Stirling's characters because Stirling managed to break free of the usual black&white morality spectrum and write a sympathetic anti-villain who opposes Nantucketer hegemony because he genuinely cares for his people and wants to build a great nation.