This right here kept me from using mine for so long. They’re (relatively, especially if you’re a subscriber) inexpensive little things, but they’re so cute and well done you want to treat them as artistically as they are. I got over that when I told myself “Ok, I’ll use the brown ones first” and then I found some nice uses / topics for the other ones, and then poof I was just reaching for one that fit the need after that. But I think also, the real thing is that their origin is in utterly unmemorable little books that were mostly advertisements and whatnot - they’re intended for the ephemeral, the meaningful-only-in-this-exact-setting sort of notes as much as, if not more than, any more grandiose type. The catchphrase I’m not writing it down to remember it later, I’m writing it down to remember it now is really golden - they’re there to help your brain take a load off. When you think of them this way, they become much easier to just enjoy and fill up with whatever you need. My daily grab book is alternating with to-do lists, grocery lists, random thoughts, etc. and I sort of “distill” anything that has longer-term value out of it when I get home and relax from the day (well, I do when I’m in my best habits, at least!). And since I like the feel and paper of some of the special editions, they’ve also found their way into my daily grind. Of course, I save all my full notebooks too, because I do like the covers, the paper, etc. And sometimes just flipping back through them I remember a time that I’d otherwise have not a single reminder of. And that’s a “good enough note” for me!
Edit: And yep, fountain pens. These are “not fancy” notebooks. They’re for ballpoints, maybe gels (except the waterproof/coated papers, which are thankfully rare). I’d be very interested in which ones you found suitable and not suitable for fountain pens (and which fountain pen ink you’re using to test them, inks themselves have widely varying compatiblities).