Soy beans, Soybean Meal, and other products of Soy are good protein sources with relatively balanced amino acid profiles - routinely used in animal feeds for that reason.
However, Soy contains some anti-nutritive properties which can interfere with digestion, most of which can be addressed by proper heat treatment (generally, this has already been done to soy meal used for animal feed). Some polysachharides adressed by enzyme additions or deliberate fermentation, and most of the phsphorus content isn't useful to chickens, passing out in the feces. Oh, and its pretty low in B vitamins, too.
Used properly, its pretty close to great. Used wrong? It has some problems. Fewer than most individual feedstuffs, but not nothing.