Well, after 3 months of using my Richmond Ultimatum in M390 steel I must say it has no peer that I've ever seen when it comes to edge retention. After 3 weeks of daily use, six days a week it would still shave. And after 3 months, a few licks on my glass hone brings it back to where it will shave a little, although not cleanly. At this point it's still sharper than my coworkers knives. I don't recall ever using anything that will hold an edge remotely this long. I think my previous champ was my Akifusa in SRS-15, but M390 bests it easily. Bear in mind that the first time I used the hone was after three months of daily use!
Even though I don't need it, I'm really thinking of adding the 210mm Artifex in M390 to my collection.
The only bad thing about M390 is that it's monotonous; it leaves nothing left to talk about. It's utterly stainproof. Nothing will cause it to patina in the slightest. Nothing seems to dull it, either. And it may be a complete fluke or maybe Mark sent me a ringer but my particular Ultimatum was the sharpest knife OOtB I've ever seen.
Standard disclaimers apply; I'm a long time customer of CKtG that occasionally does sharpening work for them. Beyond that I have no affiliation.
I hope Mark with work with Lamson & Goodnow to bring out more knives in M390. So far it's all upside with no downside that I've seen. I don't use this phrase very often but it's truly a game changer.