I have tired getting into index cards multiple times over the years, and they just aren’t my kind of thing. It all just ends up in a place where they are a thing that is better served by some kind of a notebook.
The ones Nock.co used to make was really awesome though. The paper quality was amazing, and a lot of interesting formats.
I think they are the only ones I have tried out of many that actually worked well with fountain pens.
Hopefully someone else will pick up where Brad left things off and start making and innovating in Index Cards.