Today I finally completed the 823 trilogy, meaning I now have a fine, a medium and a broad one. Two black ones and one clear one (I don't like the amber/brown colour).
If I lost all three of them, the first one I would get again is definitely the medium one. Funnily enough, my first one was the fine one, followed by the broad one, and last (but definitely not least) the medium.
Now why is that?
First I have to explain my use case: I am using my pens for everyday journaling, occasional letters/longer texts, taking notes at work (copy paper, post its, etc), and for exams (about 20 pages in less than three hours, meaning very fast writing).
For journaling I use Midori MD paper notebooks, for letters I use Midori MD cotton paper, for exams I have to make do with the paper provided (usually copy paper). I have also made experiences with Moleskine and Leuchtturm1917 paper. I am using all of them with Iroshizuku inks.
Now I'll elaborate why I consider the 823 medium superior compared to the broad and fine one.
The fine nib gives a lot of feedback, especially on MD paper and even more on MD cotton paper, however, it does not feel scratchy - very precise rather - but for fast writing, as I have to do for exams or when taking notes at work when given an assignment, it is unpleasant. It never skips, but it feels like it would definitely prefer slower writing. In summary, it feels like a very precise tool, like a scalpel.
The broad nib feels nice most of the time, but sometimes it feels scratchy. Nothing serious, not even close to unpleasant. It feels like sometimes the feed has trouble keeping up with the needs of the broad nib. This happens more frequently with faster writing. For my handwriting it is also too broad, meaning I have to increase my handwriting size to ensure legibility. I got it in clear and I like that it shows off the ink both inside the barrel and, due to the broad nib, also on the paper. In summary, it feels like a Bowie knife that has a slightly dull edge with one or two knicks that are noticed occasionally, making cutting less than effortless and resulting in unclean cuts.
The medium nib is easily the smoothest to write with. It retains a comfortable amount of feedback, but feels truly effortless to write with. All those people saying the Pilot #15 medium nib is the smoothest and best to write with are correct. It really feels like gliding. I guess the feed of the 823 and the medium nib work best together, as there is always the perfect amount of ink at the point. It is not overly wet, which would result in being slippery, but it is definitely not dry either, just the perfect amount for a nice medium line. It is definitely broader than the fine, the step up is quite big, but totally workable still. A nice line for notes, letters, and exams, maybe a bit modest for signatures (but the smoothness though). For slow, precise writing on copy paper, for example, when reviewing and amending a printed document I would use the fine one. For headlines or underlining things, I use the broad one. In summary, the medium is like your trusty pocket knife you care well for. Does it offer the most precision and razor sharpness when cutting? No. Is it big enough to take down a tree with? No. Do you need that most of the time? No. Is it a joy to use and a perfect middle ground for most everyday scenarios? Yes. The medium nib makes the 823 the ultimate workhorse pen. It doesn't require as much attention as the fine and broad one when writing. It quietly fades into the background while you write, making you forget it is there and letting you focus on your task at hand. Its beauty lies in its simplicity.
Don't get me wrong, the 823 is an amazing pen no matter the nib size. I got the fine one first and had a hard time accepting that the medium nib, the one I got third, is actually superior for my use case. All of them are great, but the medium nib is the greatest, edging out the others.

Sorry, this turned out way longer than I expected.

tl;dr: When thinking about getting the Pilot Custom 823 and you are unsure of the nib size you want: Get the medium. Also try out Iroshizuku Asa-Gao. It is amazing.