I wear a size 6 pant and bought the small. I prefer a slim fit so I don't mind, but some might want to size up for a little more room. At first I didn't like the material, but they have grown to be my favorite pants. They are so comfortable and very slimming. The logo can barely be seen unlike in the photo, which i prefer. Bought both black and blue which are very close in color. I can wear them hiking or dress them up a bit for the office.
update: I just bought the heather grey, which are similar pants, but different style from the ones i bought 2 years ago. The updated version is still slenderizing but they are much lower rise.