I had a bra like this, purchased over 15 years ago, that was worn out. I was looking for a replacement and couldn't find one anywhere. I ordered my normal bra size and this one fit and works perfectly under strapless/spaghetti straps and low backed dresses.
I will say that when I was heavier I found this style of bra uncomfortable since the tight thick elastic at the waist is what is holding the whole thing up. I used to end up with an unflattering pooch under the elastic but if you have a small flat waist you should find this more comfortable than a standard strapless bra and perfect for backless dresses.
The bra itself is pretty but the quality is a little less than one I purchased over a year ago. That one had a nicer quality lace and underwire in the actual cups but since that one is longer(almost down to the hips)when I sit it tends to ride up and ends up showing at the top of my dress both in the front and the back.
This bra's cups are stiff and hold their shape but since they don't have any boning or underwire they don't hold my breasts as securely. The elastic band ends just above the waist so there is no issue with it riding up and it is lower in the back than my other one. This bra can be worn with even my lowest cut dresses, which is what I needed.
Overall this bra fit the bill and was exactly what I wanted. The price is fair and customer service and shipping was very good.