Opus Dei (just one group among several) has too many features of a cult for my comfort.  It also preys on Catholics from poorer countries (thus its strong hold in South America, for one), and among the wealthy clergy, mostly older bishops and cardinals.  No doubt there are good and holy men and women among its members and it my well do much good in some ways, but its closed system and secretive nature is cause for great concern and even greater scrutiny.