Generally 6-Oxo is used as an estrogen blocker when you off cycle from a pro-hormone.  It can be used by itself as an estrogen suppressor and I'd rate it pretty high as far as effectiveness.
However, I wouldn't see the need for anyone to use it just because they wanted to suppress the small amount of estrogen a man carries.  It's generally better to let our bodies physiological functions to take care of themselves.
High elevations of estrogen can lead to mood swings, extra storage of fat, a harder time increasing lean body mass, etc