The premise is the King is dead and the country falling into chaos. Flendid and Smidgen are individuals who are caught up in events despite the fact one is insane while the other is a literal monster. Flendid and his companions are not heroes by any stretch of the imagination but their complete sociopathy in trying to escape the consequences of their (mostly Flendid's) actions makes for an entertaining yarn.
Flendid serves as the narrator for the book and generally gets through each scene he's involved in by digging a deeper hole for himself. He is a wholly self-interested character but his complete lack of shame makes him an entertaining cad to read about. While he had good reason to kill the woman in his bed, the general assumption by the reader is he'll do anything to get ahead in life (however bad he may be at it) and his lack of shame is why we'll enjoy reading about it.
The universe is described in a minimalist style with the names of characters as well as locations being done as curtly as possible. Never, Bush, and Claw are places. Bent, Smidgen, and the Golem are names. They fight things like the Turned. Little time is used for establishing a history of the world and what little insight we get into the setting is purely relating to how it would affect our anti-hero protagonist. The use of nicknames is sometimes good in fantasy novels, preventing us from getting lots of nonsense words, but I do think this book goes a bit overboard in that respect.
I like the handling of magic in this world since it is completely diabolical in nature. The Golem, the Turned, and other creatures are described in horrifying visceral ways with the ravages they perform being things our protagonist has no interest in stopping. I also appreciated the mystery of Flendin and how he managed to tick off so many powerful people before the start of the story.
If I had any complaints about the book, it is the fact we're dropped in media res without much context to explain what's going on. Eventually, the book does get around to telling us who all of our gang of ruffians are as well as what motivates them but the author takes a bit to get there. It can be somewhat disorientating as a reader.
In short, Since Never is a blackly funny story that I would recommend to those looking for a story about remorseless thieves with a penchant for black magic. No one is particularly sympathetic in this world and that is a selling point. As a fan of dark fantasy, I enjoyed having protagonists who are so utterly ruthless and inhuman in such a cutthroat world. This may not be the taste for everyone but it's good that the book openly advertises itself as grimdark.