People buy different notebooks for many different reasons, but most of it can be boiled down to the following: use case, paper properties, how it looks and the format.
How a notebook looks is very important for some people, while others think it is less important. I personally prefer to either use a cover like the Midori Travelers Notebook or to go for a classic design like Leuchtturm1917 or Field Notes. While others go for something with something fancy with a lot of colour or even a themed notebook. Some people even put stickers on them. It can be, like everything else, a way to express to people who you are.
The pens and inks you are going to use with the notebook is a very important factor when it comes to what kind of paper you want. It is both a question of aesthetics and practicality. It looks horrible when the ink bleeds through from the two previous pages and your writing feathers like crazy. The kind of dry time you can tolerate is also an vary important factor.
The format is probably the most important part of a notebook. It is both the size and dimensions of it; but also how the pages are designed. You have the three classic layouts: grids, lines and blank; I know there are many different kinds of grids: boxes, dots etc, which you prefer is up to you; but they are more or less the same thing. This is closely related to the use case.
A use case is the thing that combines the three previous parts together. How a notebook looks might be important depending on where you are going to use it. A classic Leuchtturm1917 is probably a better fit in an office than some crazy porn themed Moleskine copy. What you are going to use it for is very important when you try to decide what kind of layout and format you want. Something you only use at your desk will probably be very different from something you carry around everywhere. For example, I mostly use A4 notepads at work, while my Midori Travelers Notebook is the notebook I bring everywhere.
How the page looks is important for a number of reasons. You probably want something very specific if you are going to use it as a planner; or anything else that is tied to a specific use case. While other times you want something more generic. Some people prefer a blank page because then they feel like they can do what ever they want with the page; and others prefer it because it looks better. Others prefer a lined page; like myself; it is more convenient when you only write, and it wastes much less space than a blank page. Grids is a very interesting layout, most people I know, that prefer them, do so because they get the flexibility of a blank page, with the infrastructure of a lined page.
What is it that makes a notebook useful or not? There are two factors that play a very important role for me. It have to either be portable(pocket sized, Midori Traveler Notebook sized or A5 sized) or A4 if it is something I’m just going to leave at my desk. And then you have the most important thing. How long does it take any of the inks I have in rotation to dry? The last thing I want to think about when I either take notes or write is to wait for the completed page to dry before I turn the page.
This makes some of the more popular notebook brands more or less useless for me in a day to day context. Both as a lefty, and as a person who don’t enjoy to wait 30 seconds for a page to dry. It should be dry within a few seconds, I can stretch as long as five.
I’m going to do a proper test of some of the more popular stationary geek notebooks as soon as I have the time. My goal there is to look at how useful they are. Rhodia make great notebooks, and they are especially useful for testing inks. But they are not usable for me because it takes way too long to dry, even with a non fountain pen.