The fabric is something like jersey fabric - it's very soft and slouchy/stretchy and will give you a lot of freedom to move your arms. It's very slightly looser than I expected (M - I tend to expect M's not to fit me as I have broad shoulders and back) but that made it so much more comfortable to wear. I had it on the entire day and until I looked in a mirror I completely forgot I was wearing it on top of a sleeveless dress. Fabric quality seems pretty decent, but I've only worn it for a day and not washed it yet.

Only grouse (more with my body than this): I have a short waist and the place it hits isn't exactly the most flattering length (somewhere RIGHT above my hipbones, making my waist vanish), but for people with normal-length torsos it should be fine. And it still looks pretty nice.