I'm going to take the board at its word and post here even though it's been ages since the previous. Hi!
I'm just re-reading M@A, mainly because of the announced Watch TV series, and being interested in working out how I think I'd go about turning the material into a TV serial (and idea I'm completely obsessed with right now).
I've always regarded M@A one of the strongest Watch stories and a lot of the critisisms raised on this thread it simply never occurred to me to consider, so this thread interesting reading.
I think of G!G!, M@A and Feet Of Clay as forming a trilogy of their own, the story arc that covers all three being the story of the Watch's, and thereby Ankh-Morpok's, development. That topic has been satisfactorily explored and brought to a close by the end of FofC.
(So when we next visit Vimes et al after FofC, it's in a new kind of story for the Watch: the story in Jingo, The Fifth Elephant and Thud! is about how Ankh-Morpork relates to the world outside itself, I think. Night Watch crops up in the middle of those, but stands alone thematically. It explores new territory rather than speaking to the existing themes of the Watch Books.)
But in that first trilogy of G!G!, M@A and FofC, the conversation is about the relationship between the the Patrician, the Police and the King. Both the characters that are these things, and the institutions they represent. And how they relate to the city. I think those three books are Vetinari's story as much, almost, as Vimes'. Vetinari works best as a seldom-seen, enigmatic figure. One certainly wouldn't want him to be a viewpoint character whose thoughts we could see (apart from that one time in FofC. But he wasn't himself). But each book sees the plot revolve around Vetinari: deposed in G!G!, shot in M@A and poisoned in FofC (I'm actually starting to feel sorry for the poor ol' tyrant now...)
And of the three, M@A is the book that really explores the Vimes/Carrot/Vetinari dynamic, and I love it for that reason amongst others.
It's true that actually Vimes doesn't have much to do in the book - I always get the impression he does more but on revisiting the book, he does basically spend the first two thirds sulking and falling off the wagon

Carrot is the more active character in M@A and I agree with the people who see this book as a crossroads for the Watch: Carrot has already developed hugely from his G!G! persona, into someone who is still simple, but can use simplicity in complex ways. He is becoming an interesting character, and I could totally imagine him going on to front the rest of the Watch books in an alternate trouser of time. Vimes would surely be his Beleaguered Police Chief in this universe. But PTerry took Vimes forward as the lead instead, and consequently Carrot's character development was somewhat stifled. Carrots probably at his best in M@A and FofC. If he'd continued to develop along the lines we saw there, though, there'd be no room left for Vimes to operate. Carrot has to stay somewhat simple and innocent to be Vimes' foil.
If I had any critisism of the book it might be that Angua strikes me as a bit of an inconsistent and flat character, a problem I feel she never quite recovers from. I don't dislike the character (I like her best when she's in a supporting role rather than point-of-view), but - well, in the way some have described Carrot, I don't think she's ever allowed to develop beyond being a fairly flat character. Her traits are a. werewolf b. girl.
I love the closing third of the book particularly. I find Cuddy's death and Detritus' response to it very affecting. I love the drama of Vetinari and Carrot being shot. Actually, I love Detritus and Cuddy throughout. Colon and Nobby are also great in M@A.
I like the gonne plot; I never had any confusion with regards to whether the thing was sentient or not. Sure, it's ambiguous in the text - it might be talking to its wielders, it might just be their own subconscious. I don't feel its detrimental to the book that the point is left ambiguous. It's an example of what the Discworld always does: takes an idea to its fantastical - but logical - extreme. A gun represents unearned power (you don't need any skill to wield it etc). It's a temptation to people who would change the world for whatever reason.