In reply to DeSlacheable.

Portability and ease of use are the two biggest benefits. I tend toward the minimalist perspective in terms of layouts and logging, but I've yet to worry about messing up one or more pages in a notebook because if I screwed up drawing a spread or put something on the wrong page or in the wrong order because I can simply throw out a card and start over very quickly. (I don't suffer from the dreaded perfectionist perspective as a result.) For those who prefer more decoration/artistic space, you can also go toward larger 5x8" cards and not need to worry about the growing binding or notebook expansion some see with washi tape and stickers. I find 4x6" is a pretty convenient size, especially when folded in half for putting in my pocket.

Index cards are incredibly inexpensive (especially in bulk) compared to the more expensive notebooks, but one can definitely find higher end, thicker cards (Exacompta Bristol cards from the company that makes Clairefontaine comes to mind) if you want something more luxurious or fountain pen friendly. Most cards seem to do pretty well with fountain pens in my experience though. I only ever write on one side (for convenience and more easily perusing my collection as well as speed of never having to worry about hidden information written on the backs of cards), so ghosting and bleed through are never a problem. It's usually not too difficult to find index cards printed with a variety of lines, dots, grids, etc. or I may occasionally custom print or cut my own when necessary. If you dig around you'll also find companies that specialize in (more expensive) preprinted layouts like Jeff Sheldon's Analog/Ugmonk cards, Notsu cards, or 3x5 Life, but I like the freedom to switch up my layouts or expand them if I'd like.

Another thing I love to be able to do is lay out handfuls of cards on the table and order/reorder them as necessary (especially my project cards and associated notes). I've also picked up a couple of note card "bleachers" for being able to store and view a handful without taking up desk space. Playing card holders can also work well for this at a cheaper price point. In my case the lower bleachers are the higher priorities while the back benchers are lower in priority, while still being visible.

Definitely going to continue this for its simplicity and functionality. I'm currently working on restoring a larger filing cabinet for longer term practice and more convenient storage, but there are lots of other options in terms of style and size out there from simple and cheap to more lovely tactile wooden experiences as well as small desktop to atomic era bunker-style furniture.