First of all, Amazon doesn't list the sizing chart, but I found it on another website:

Small  = band size of 32"
Medium  = band size of 34"
Large  = band size of 36"
X Large = band size of 38"

(I usually wear a 34B, and the medium fits me well and isn't too tight. My inexpert opinion is that it would also be fine for someone with C-cups, but might be a bit tight for anyone larger than that.)

Details: The bandeau has elastic on the top and bottom and two sets of hooks on the back. The hooks are great- they should keep the elastic from getting too stretched out, and having two rows adds some flexibility to the sizing.

The back panels are just lace, the front is lace over solid black. It has removable pads and places to attach bra straps (straps aren't included). It seems pretty secure even without adding straps, but it's nice to have the option, especially if you're worried about it shifting around or want more support. It's also nice that there is no clear center, so it'll look fine even if it moves a bit to the left or right.

Overall, I really like this piece. It's exactly what I needed for layering under low-cut dresses and tops, and it has a lot of convenient little details I haven't seen in similar items. I haven't had it very long, so I can't speak to the durability.