I love the IMI M855. I still think XM193 and Q3131A are probably ballistically superior for short range engagement of humans for defensive purposes, but M855 is still a good round, and the boxed IMI is first-class high quality stuff. I think I probably trust it more than the XM193/Q3131A in terms of QC and reliably going bang every time. I have purchased it in the past both as a case of 1200 (original factory cardboard case with IMI markings, 40x boxes of 30), and I've also gotten it in as a sealed metal ammo can of 990 (33x boxes of 30, with some styrofoam inserts to keep it packed tight inside) that is stamped up as IMI M855 on the outside as well.
I keep a mix of IMI M855, XM193 (not PD), and Q3131A for my bulk SHTF supplies, in addition to a smaller quantity of Hornady 75gr TAP (which I feel is superior in every way, just much more expensive to stock up on - but it's what I keep loaded in my mags, and I wouldn't dig into the SHTF supplies of the other stuff until the TAP ran out).