I read about 20 chapters, so bear with me.
The art is subpar. Compared to Death Note for instance, it's night and day.
The main character is uninteresting. She is gullible, yes, but gullible doesnt mean retarded. And she went full retard several times. I know her and the con artist are supposed to be polar opposite, but it just doesnt work.
So far, the story's decent, but it's been poorly introduced. You can keep us in the dark all you want, but you need to give us something at some point, or we are losing interest. I'm 20 chapters in, and i've no clue what's going on. There's no sense of scale.
At least, where i stopped, the author gave us a peek in the con's past... but it's too cliché. I wont spoil, it's just cliché.
I dont understand the intentions of the author, and if in 20 chapters you cant even start to guess what's going on, then what's the point?
I'm hesitating between 4 and 6. I think it depends on what you're looking for in a manga. So, for me, it's a 4.