I'm 5'8", 120 lbs. bought the size 4. They are a perfect fit for me and I like that the waist is a little higher so nothing is showing in the backside when stretching or bending over. I thought the legs were plenty roomy - however, I have fairly thin legs and I am not very muscular. The fabric is nice and lightweight and the legs zip off easily. I am pleased with this purchase.

These pants are awesome, except they do run small. I wear a size 4 (or even 2) in Prana, and a size 4 Colombia & REI pants. So I thought it'd be a safe bet here. 6 would probably fit better (I am 5'7'', 135lbs, small waist with muscular butt and thighs), the 4 did fit perfectly on my waist but too snug on my butt/thighs.

I am 5'3" 115 lbs and ordered the size 2x32. The color "shark" is a very dark gray.
Pros: They fit me exactly. They look great - no weird bulky or tight spots, no saggy crotch, no back gap when sitting/leaning forward. They are extremely lightweight and comfortable. They are long enough that they cover my ankles when I am sitting. When converted to shorts, they look like regular slim shorts that stop just above the knee. The side cargo pocket is big enough to accommodate a phone (and more) and has a Velcro closure. This pocket is pleated such that it doesn't bulge out when empty.
Cons: as with all convertible pants, the zipper at lower/mid thigh is a little uncomfortable/stiff feeling. The back pockets are small and have no closure - I wouldn't use them for money or a phone. All of the button holes (one at the waist, one on each lower leg to hold strap for rolled-up legs) are too small for the buttons, but you can force them a couple of times (requires strong fingers) and they stretch out. The front zipper (not the leg zipper) is so short that when the pants are fully unzipped, they *barely* fit over my (quite petite) bottom & hips to get on/off.
Overall once they are on, they fit so beautifully and are so lightweight, I would definitely buy these again.