Overall:

The low-back configuration gives you that "I'm wearing a proper bra" look without actually holding your twins in place.

I purchased this so I could wear this dress[...] with a bra. When I go the bra, the packaging was nothing special. Even though the form struck me, the big thing with this bra are all the straps, so if you're one of those that can't keep your bra straps straight, I would not recommend this for you.

The way the low-back configuration works is by having a very long strap that crisscrosses at the front and clasps at the back. This means that the bottom-front of the bra may pop out a bit, depending on how tight you have it and if you buy a size a little larger around, so also keep that in mind.

The bottom strap didn't really stay in place, I found myself adjusting the bottom strap to go over the bottom-front of the bra several times throughout the day and checking the back because ti didn't give me the confidence that it was actually hidden at the back.

As far as the twins, they were sort of falling out the front a lot. I am ~32C, and I believe I got a 34B so that might be why. But in general, the ladies would bulge out at the front a bit and I'd have to adjust them every hour or so.

I can't speak to the other configurations yet. I'll update it once I can.