I used Custom 823s exclusively as my work pens for several years. I love the nibs, the extra large ink supply—about 2.5 ml with the inverted fill technique—and the fact that it is a demonstrator. I could just grab a pen in the morning, hold it up to a light, and instantly know if I had enough ink for the day. They are also my travel pens. With the ink shut-off valves, one can be fairly certain they won't leak with pressure changes. I do keep my pens nib up in my shirt pocket, though, and open the ink shut off with the nib up, too.
I also like the vacuum filling system. At one time, I was trying to acquire at least one example of all the commonly available filling systems. The Custom 823 was a well-made example of a vacuum filler.
Pilot USA has great customer service. I once gathered up about 20 Pilot pens in various states of disrepair—I do use my pens at work! I sent them all off to Pilot. There were bent nibs, broken finials, leaks, the usual gamut of pen maladies. About six weeks later, they all came back, repaired, parts replaced, tuned, and writing as well as ever. And at no charge. Try doing that with a Montblanc!
For the most part, if a pen manufacturer makes ink, I use that ink in that Manufacturer's pen. I believe that they were made to work together. Pilot makes a lot of nice inks. Their black is a well-behaved dark black, blue is a nice medium blue with good properties, and blue-black is a reasonable blue-black. They also have the Iroshizuku line, so there are many colors to choose from.
I do like demonstrators; I like to see the inner workings of a pen and I also like to know how much ink I have, in an instant. But even if you don't like demonstrators, the amber Custom 823 is fairly dark, especially when filled with ink. At first glance, one wouldn't even know it was transparent. The smoke 823 is even darker.
I don't mind finding a pen I have for a lower price later on. If I like the pen, I buy a few more at the new price. That way, my average cost per pen goes down. And I have more pens.
Thank you, Sonik, for posting this this thorough review of one of my favorite pens. I agree that it is in a "league of its own".