I've been reading it for awhile now, among other things. I have the two three-novel collections that feature a few short pieces and the three novels that came afterwards. Just finished novel 8 'The Last Ditch' and making my way to novel 9 'The Greater Good'. It is basically 'Flashman' meets WH40k. I found the 1st 2 novels to be the best and a distinct lessening of quality in novel 6. I don't remember novel 9 exactly but I did read it and don't remember being impressed (hopefully I'm mistaken).
These stories are decently written but I wouldn't think they'd be as enjoyable or nearly as funny to anyone not fairly familiar with the WH40K setting and the novels. The setting and other fictional work, is of a grim and repressive far, far future. The Ciaphas Cain series makes sure that the reader understands that that future is also British. It seems to rest somewhere between the TV series 'Rome' Britishy Romans who are perfectly cultured and brutal and Python's 'Life of Brian's Britishy Romans making you conjugate Latin (or they'll cut your balls off).
I think the best book is the first so certainly the place I'd recommend to start with if anyone wants to give the series a try.