I am currently using a Cahier Journal for my work-related bullet journal. As a fountain pen user, I hate it. Unfortunately, I bought the 5-pack from Costco and well, I need to use them for something. I tend to blow through work-related journals so these's that. Why did I buy them? They are offered constantly here, and they are mentioned a lot in online fountain pen communities so I figured they couldn't be all that bad. Oh, blissful ignorance, what a horrible blow you have dealt me.
Ink takes long to dry on them, there's bleeding and ghosting, the paper feels flimsy, etc. From what I've read online, there are some inks that do make them usable (such as Diamine Regristrar's and Noodler's Legal Lapis), but unfortunately I don't own any of those. It is absolutely horrible with my favorite ink (Noodler's Kiowa Pecan) but is passable with Lamy Black. I hope that I blow through them soon and will stay the heck away from them in the future.