I really like it. It fits true to size, and it's way more comfortable than I thought it might be. The elastic is wide, which lets it sit comfortably without digging in. It spreads out across my back and loops comfortably around my mid-section to velcro closed just under my bra strap. I'm used to something sitting there anyway, so I barely notice it. The strap around my shoulders is the only place I can really feel it. It's also thin and flat and can't be seen through clothing. Obviously a difiference in posture can be seen, but the brace itself cannot be.
Really all it does is gently nudge me to drop my shoulders down and back, which then makes my whole spine straighten and elongate. It's hard to roll forward and slouch with your shoulders back. It does this through the mildest of discomfort. The elastic just gets a bit uncomfortably snug, if I roll my shoulders or hunch forward. I absolutely still CAN, but it's more comfortable if I don't. Being as most of us are vaguely hedonistic even if we don't like to think so, we tend to do the things that make us more comfortable, which in this case is adopt a healthier posture. It's a very clever way of going about it. It obviously only works if you want it to, but if you didn't want it to, you wouldn't be here.
I do completely believe the included paperwork that states this will be uncomfortable for a week or so, though, and to take it slow and gentle. The muscles need to adjust.