0.95 - 1.20% carbon (very good edge-holding)
16 - 18% chromium (excellent rust-resistance)
^These are the main things that impress me about it. Takes a very fine edge and is able to hold it well, AND it doesn't rust easily at all. It seems to perform well over a pretty wide range of hardness (mid-to-high 50s, at least). Boker has used it in some of their traditional line, and a bit softer (reminds me of Victorinox's stainless). On the other hand, it used to be very popular among custom and high-end production knife makers, and at pretty high hardness, I think. Either way, it seems to perform very well.
Aside from that, I also like how easily it sharpens up. Responds excellently to silicon carbide and diamond hones, and polishes beautifully. I have an older Buck 112 in 440C, convexed on wet/dry sandpaper through 2000+ grit, and I love it.