To get the obvious out of the way first: it's a monster-of-the-week story, with a resolution that effectively hits the reset button so that the world is not much changed after the events described. But I think it is a very good story of its kind. The Sin Eaters derive from Christian theology, and infiltrate their victims via a special form of baptism, with inevitably nasty consequences. It would be very easy to write a very stupid religion-and-Torchwood story; but Minchin confidently takes Gwen and Rhys through matters ecclesiastical, in what for me is the slightly foreign church environment of South Wales.
The brutal reduction in the core cast at the end of Torchwood's second series means that we have only Jack, Ianto, Gwen and Rhys left as central characters. There is a neat contrast in the relationships - Gwen/Rhys, married over a year now; Jack/Ianto, still at the frenetic fumbling stage; and also Rhys's friend Matt whose stag night provides the catalyst for the story as a whole. Given the extra space (2 CDs of audiobook is worth several Torchwood episodes) we get a lot of decent exploration of love and religion in the world of Torchwood, though I felt that the author would gladly have given us more if space had allowed.
Also, Gareth David Lloyd is an excellent reader, switching easily from strong to weak Welsh accent depending on character, and also doing a good job on Jack's American. He is recognisably Ianto from the start, but (of course) more so when in character, and also carries a conviction that lifts the various character moments and vivid descriptive passages tremendously. I was disappointed by the first Torchwood audiobook I tried, and probably wouldn't have bothered with this one if it hadn't been for the family connection, but I will try some more in future.