If anything, soy protein seems to be good. I am planning to try it -- lately I have not been taking enough, so I am planning to use soy.
I won't cite any pubmed study abstracts here unless someone asks for one, but from what I have seen, one can summarize the benefits as follows:
(1) SPI decreases estrogen in males
(2) It does NOT decrease the level of testerone in one's body
(3) It is more or as anabolic as casein
(4) It is slow digesting, like casein.
(5) It is more lipolytic than casein (per calorie).
(6) It is anti-carcinogenic (as WPI is).
Some of these properties have been attributed to phytoestrogen, and some to isoflavone.