M390 is phenomenal, I have it in a Contego KW exlusive. Edge retention is better than any other steel I have, it lasts at least 50% longer in normal use compared to ZT's Elmax, which is my runner-up for edge retention. It takes an insanely sharp edge pretty easily as well (much easier than my D2 or Elmax), and I've never heard of anyone having it rust.
It's easily my favorite steel for folders these days. Shame tends to cost so much or I'd have a lot more of it.