I ordered a size up after reading reviews but I feel i should have kept to my normal size (small) after all. The medium fits fine but it rides down and up a bit more than I'd prefer.
I've worn them with jeans and dresses and you definitely cannot see the seams - which is great! I haven't tried them with slacks but I think they'll be just as seamless.
The fabric quality is nice, too! Soft and very comfortable!