My search didn't stop with the .5mm pen. I also wanted a pen nib that was smaller than that. Ultra fine or whatever label they put on that. I tried a bunch and I'm not sure I love any of them.
I started with the Pentel Clena .38mm. Since I love the Pentel EnerGel so much, I was hoping for lightning to strike twice. It didn't. The pen is okay but felt scratchy writing. That's one of the problems with the fine tip and ultra fine tip pens. I could use it, but it wasn't an enjoyable writing experience.
Next up, the Pilot Juice Up in .3mm. Very meh on this one. Writing was very scratchy, the colors were difficult to see because of how thin the line of ink was, and I just didn't enjoy using this at all. I do have an order of the Juice Up in .4mm on its way. I'm hoping that's better.
The Uni-ball Signo 207 in .38mm weren't bad. I could use this pen. It was among the best of the ultra fine tips I tested. I'd recommend it with the caution that you try it out for yourself first before committing to it with a 12-pen box.
The pen I liked the best in this category is a little expensive. More than $4 for one pen. Gah! But it's the one I liked best. Korea Life&Pieces 0.38 mm wrote smoothly and was pleasant to use. This is my favorite choice in the ultra fine category. I got an olive green ink pen to use in my Hobonichi Cousin since the cover I bought for it is green. If this pen wrote as well as it did in olive, I'm curious to try their black ink pen.