I did some volunteer work for Opus Dei a few years ago, but I didn't join. You characterization is incorrect. While there are some wackos in any organization, for the most part Opus Dei is pretty much plain-vanilla Catholic. The mainstream perception of the group is unfortunate and unwarranted.
I have known three Opus Dei members. Two were obsessed zealots one with psychiatric disorders ( whom Opus Dei recognized and was sent for pastoral assistance). One was as you say a "plain Vanilla Catholic" One was a vulnerable person who needed to belong to something and be special. Opus Dei filled that need.
I rank Opus Dei along with the Liberation theologists. Fringe types. Both OD and LT's are perilously close to cults...
When I say Brownshirts and Che Gueverra in Cassocks. I'm engaging in a bit of illustrative hyperbole of course. The phrases came to me from Franciscan priest at St. Francis University. I enjoy the banter of priests because they can be very creative with language.