This stuff frankly sucks to walk through in sandals, well in anything other than knee high leather
boots, actually. Animals generally don't eat it, but horses can develop a taste for it, and this will cause a serious illness that leads to death. It's a fairly common
hay contaminant, but people have gotten to be more aware of it lately (100 horses died in 1954 in california from ST poisoning). You'd have to have an incredibly tough mouth to get past the thorns.
http://wric.ucdavis.edu/yst/impacts/impacts.html 
Bees seem to like the flower. The UCDavis site says that it's an important
honey flower in California, actually.
It seems to be one of those plants that gets blamed for ruining pastures, but from what I can tell it comes in when the grass is sufficiently weakened from over grazing. That's the case with most plants blamed for "ruining pasture."
It appears to prefer dry compacted surfaces, but I've been warned by neighbors that if you
water it in an effort to "drown it out" it will just get huge. It's one of those opportunistic plants, moving in when other friendlier things have been eaten down too many times. It pops up in "waste sites," along roads, and in abused agricultural fields.
And, it makes bare feet or even bare legs really intolerable! How to deal?
It's only in a few spots in our field (the places that were skid sites for logging and have been compacted with machinery), and we haven't done anything to deter it thus far. This summer we're going to start cutting it before it seeds. Letcha know how that works. We also might burn the area with seeded plants? Or smother mulch for a year or two?
I didn't find any information about the alleopathic qualities of it, but I don't see much of anything else at all growing where it grows, that speaks volumes.
