I listened to the audio version from Isis, narrated by Nigel Planer. This is one of my favorite Discworld novels, as it introduces several characters who Pratchett develops in interesting ways in future installments of this going-on-thirty series. I've never listened to an audio version and it was great. Maybe it's only Americans who feel this way and Brits can already hear the accents in their heads, but it really adds an extra layer to characterization to know that Nobby sounds a little bit Cockney and Carrot sounds a bit gormless and Vimes, for some strange reason, has a cold.
I don't recommend Guards! Guards! as an introduction to the Discworld (because why? I dunno. It's just not the easiest. Suggestions welcome. Maybe Mort). But it's funny and clever and has some great subtle commentary on human character and on the nature of government. If I ever have a male pet, I now plan to name him Errol.
There was one drawback. Pratchett's writing tends to be very episodic. The narrative jumps frequently between multiple plotlines, times, and locations. It's easier to keep track of this when there's the visual cue of the chapter break or the big expanse of white paper between chapter sections. Listening to the audio book, I often missed these transitions.
In sum: read the Discworld, but don't start with this one, and if you want to listen to them on CD it's loads of fun but limit yourself to ones you've already read.