My opinions on those books aside (I hated Ronin for example), he's still not consistent enough for me to class him as a great writer (more of a five-hit wonder if anything). We have different criteria for what makes a good writer, but to me its either somebody who either pushes themselves to create/explore something new/different with each creation (which he stopped doing about a decade ago), or somebody who continuously hones their craft to the point of storytelling perfection while not re-treading the same thematic ground (which he also stopped doing, when he started falling into the same patterns over and over again). Its a shame, because he's definately a creator who had a lot of promise, and he used to have a lot of passion for the medium as well. He's a lot like Stephen King or Michael Crichton, who have both written some amazing books, but fell into a lazy cycle at some point in their career (probably when the money started to roll in).
His artwork swings wildly as well, sometimes its very well done (300, DKR, some Sin City), other times its weak (Ronin), and yet other times its downright lazy and sketchy (DKSA).
My personal problem with him is that he always writes the same (or similar characters). His heroes are sadistic emotionless MANLY men (Marv in different outfits), who ALWAYS speak in gritty monologue. His villains are effeminate girly men, or their emotions are perceived as a weakness (in fact any character who expresses their emotions is written as a coward, a fool, deserving of a beat-down or at least complete contempt from every other character). If he writes a heroic woman, she acts exactly like his men (cold, emotionless except for anger, sadistic), except with pronounced butt and breasts and worrying posture. Usually she'll be a prostitute or stripper, and she'll always be talked down to by the men (and she'll damn well like it!).
If its meant to be satirical, then he's only satirising himself, which is a bit *********ory.
He comes across as insecure, misogynistic, xenophobic, homophobic, and downright lazy and complacent.
I do LOVE a handful of his work (Year One and DKR were among what got me into comics in the first place), but they seem like flukes when you compare them to the crap he's producing now. Its like when a great band gets old and starts going downhill, sullying their own name in the process when the fans realise that what they hate in the new stuff (repeating formulas and characterisations in Miller's case) were always there in the old stuff, they just never noticed it until now. He's a hack writer who has written some great books.