Sterling, it is not completely opaque. I'm comfortable wearing it over just a bra and no camisole, and you know I'm pretty conservative about that sort of thing. But you can see the basic outline of the bra through the fabric if you look. If you're looking for something that's 100% opaque and doesn't show anything at all, I would pass.
The J Crew version is so cute - of course it's already sold out in anything above a 10. Sigh.