This is unquestioningly a horror novel. There are some scenes that are, frankly, stomach turning. Most involve Bertie and/or feces in some way, yuck (to both of them). But that's the type of book this is and it shouldn't be off-putting. If you pick up a horror novel, you should expect some gore and horror. You get plenty of both in Flux.
The book does drag at times, but it isn't enough to put you to sleep, and does serve to highlight the action in contrast. There are some really interesting characters here. Iain's two best buds I loved instantly. Beer guzzling, moderately aimless lads in their early twenties who, if I was in my late sixties, I would describe as 'good boys.' Though less ostensibly horrific, I felt sorrier for Dave than Iain. You'll have to read the book to know why.
This is a classic novel of light versus dark, good versus evil, sane versus insane. Possibly more the last than the first, but that's an ontological question that I think the book forces you to address. It keeps you guessing, you're never sure what will leap out at you (and poor Iain) next. I think it is well worth picking up, and if you are a particular fan of horror it's one you won't want to miss.