So, why am I fooling around with a new type of re-positionable binder, when I'm a "dyed in the wool" Filofax sort of person? Well, it's all about ISO standards and, in particular, the requirement for punched holes to be positioned 11-13mm away from the edge of the paper. Which means that, perhaps miraculously given the incompatibilities in life these days, you can use a standard hole punch to perforate each ARC page to enable them to fit Filofax binders.
Currently, here in the UK, Staples are knocking out packs of 50 sheets of lined 100gsm paper for two quid in both A4 and A5 formats ($3.99 and $2.99 respectively for letter and junior formats in the US). And this paper hits the spot when I want to use my fountain pen. I can't vouch for every pen and ink combination but my Lamy ST, with a medium nib and black Sheaffer Skrip ink works just fine on this super smooth paper. I'd be inclined to blot the last few lines before you turn the page, though, as drying times are not particularly fast.
The ARC punch isn't in the UK at the moment but, when it is, you will be able to punch your existing A4 and A5 Filofax inserts too, so enabling all sorts of possibilities; like, for instance, the option to keep an ARC binder on your office desk but travel to and from work with that lovely A5 Malden you simply can't risk losing, transferring projects between binders as required.
Ok, this may not be binder nirvana, but the concept of combining the two binder systems is definitely worth thinking about.