This, I think, is a solid 3.5 stars. I would have love to give it a 4 but I think despite the writing style and characterisation, something still fell short.
When I figured out the main couple/relationship trope - assassin chemist girl x borderline psychopathic but morally good guy - I was hooked. I think Sera was an incredible character, and I loved seeing her in all her badassery. And of course, who wouldn't love Cassius. He was perfect .... though, some might say (me) he might have been too perfect at times. Things seemed to go really well for Cassius, all things considered. Nothing ever really went (too) wrong for him, and he always came out as winner in whatever he does. With his brain condition making him a complete daredevil, while also being this Prince Charming type, Cassius lacked flaws that would have made his character have a lot more depth. Nevertheless, his character remained interesting till the end.
My only one gripe with the book was that it wasn't as dark as I thought it would be. Sure, it was quite gory and there were plenty mentions of blood. But in terms of questionable relationships, Cassius and Sera were pretty fluffy most of the time. In fact!!!! It wasn't smutty either. They have like two major scenes, and then one that was like ... closed door. I can't lie, I was really expecting something more.
In all, I think this is a good book to get you out of a slump. It's pretty short, so you'll breeze through it and the writing style is enticing enough that it's both simple and beautifully done.