I'm 5'6", 160 pounds (not muscular), ordered an XL.
These are comfy and I like the pattern. They are a nice weight for summer but they are still thick enough that they aren't see-through. The main positive for me is that almost every pair of workouts pants give me terrible camel toe but these don't, which is obviously a plus.

I have two big issues that make it hard to wear these to actually work-out in. The first is that they fall off me when I run. As in I literally had to hold them up while we were running laps so they didn't fall off. The other problem is a have a long torso and these are just too short in the waist for me. I feel like I constantly have to adjust myself so I don't have plumber's crack, which is not what I want to be worrying about when I'm bear crawling. Part of the problem might be that I have a pretty big bum. Maybe someone who looks like Paris Hilton instead of J.Lo wouldn't have as many issues.

I think they would stay up better if I tried a smaller size, but that would have made the problem of them being too short in the waist even worse. I wear them for low-impact exercising at home, or as my change of clothes after a race. I won't be wearing them to the gym again though.