I'm 5'7" and 140lbs and the small fit me perfectly.
This is my first one piece triathlon suit, and I am a convert! The flat and wide arm/thigh holes are super comfortable, no pinching, binding or riding up. The chamois is the perfect thickness to provide comfort without feeling like you're wearing a diaper.
And since no triathlete is complete without a tiny bit of vanity...the way the black and white is blocked on the suit serve to make you look super thin!
You can't really see it in the photos but there are two small pockets in the back that would fit gels or whatnot. They have flaps so nothing will fall out.
I was skeptical about spending this much money on a trisuit but I'm so glad I did. WAY more comfortable than my old tri tops that would ride up on the run.
The only potential con (though I like it) is that the white fabric on the side is a little bit transparent. Not really bad, and it's just along your side so nothing scandalous is showing. But I have a tattoo on my side and you can definitely see it through the white of the suit.