I really love my running tights. They provide a lot so support around my hips, thighs, hamstrings, and it seems I can run longer without feeling the normal fatigue that I usally do. I fell between a XS and S on the size chart and chose the S. They fit fine, the XS may have been more constricting, but I'm sure would have fit too. The small's crotch works its way a little low while I run, hence may be a little big.
On the other hand, I purchased the CW-X tri shorts from another company and this small size is pretty small indeed. I may have been able to even wear a medium.
Overall, I love the way they feel. My husband owns the CW-X tri shorts and he finds more stability than his regular tri shorts. We like the brand a lot!