They're a must have for a strapless dress, but they can bring their own set of problems. They're forever wanting to slide down. In formal occasions, where I might need to wear one, I'll use a little garment tape to help hold everything in place.
I hate strapless bras, I need new ones actually the one I have never stays up at all it always falls down and I find myself picking it back up. I need like an adhesive bra or something, something will hold me up. Its so annoying when you have continuously pull it up.