So it has a lot better edge holding? Is it brittle and am I going to be able to sharpen the thing?It isn't just Benchmade. Spyderco, Kershaw, and others have made M390 knives. The stuff is difficult to grind at high hardness, i.e. costs more to manufacture.
If you don't cut a lot of abrasive materials, and if you're not a collector, then M390 might be an unnecessary expense.