In addition to that, I gradually switched to their 0.38 pens of different colors. With fountain pen it was more stylish but impractical for multiple semantic(*) colours. With cheap, light, yet nice quality pens of different colours I can not only carry multiple ones, but have the same set at multiple places.
(Dark blue is body text/drawing, light blue is comment/secondary annotation, orange is action/process/message passing, green is data/metadata flow, …)
Only downside I’ve found is the paper is thin enough that it’s not a great notebook to have at a bar, where you might get condensation dripping on it, nor for the gym where your hands might be sweaty. For the same reason it’s not much of a sketchbook, but for writing it’s my go-to these days.
I like the pens well enough to keep some around, and I appreciate all the color refill options, but I don’t think they’re “better” in the same way the notebooks are.
I think half the time I go to a Muji store it's really because the entire space gives me a sense of calm. Making a brand that's inexpensive but still projects a quiet feeling of quality and attention to detail is rare, at least in America.