If I remember right Benchmade switched to x15tn after they decided that the H1 steel they were using in their dive models didn't have enough edge retention for their taste. X15 is supposed to have better edge retention at the cost of being less corrosion resistent, but still signifigantly more rust resistant than the majority of steels. I think its a French steel in origin.
Its a good steel, holds an edge ok with BM's heat treat but I wouldn't put it too near s30v, more like a solid AUS 8 in my experience.