I really like these socks and have ordered extra pairs a few times already. I noticed they have recently changed design a bit, but still look nice and feel great - the new design is a slight improvement I think. On the positive side, the socks are light, very thin, low profile, and very comfortable. They are also great for things like pilates class, where I prefer some nice looking low profile socks, rather than going barefoot. Also, on the positive side, these socks have survived longer than the "TwinSkin" version, which literally had a hole on my big toe after the first use.
On the negative side.. my oldest pair of these socks has just now developed a small hole, and that is probably after about 10+ very sweaty and rather intense gym sessions. I have been doing things like "body combat" gym classes in them - but haven't really used them for any serious running. The running I have done in them was fine - they were better than other socks in terms of avoiding blisters, but perhaps not the best I've had overall.
Also, I've noticed that these socks do seem to shrink quite a bit after use and a few washes. I don't machine-dry them - they are light enough to air-dry pretty quickly. I wear a size 11 shoe (US Nike Size), and I have bought size Large in these socks - but they still shrink to just barely big enough. Which I don't mind - except, as other people have noted - once they are shrunk a bit, they tend to slide down under the heel when they get very sweaty, and then I can find myself in the middle of my gym class with my socks under my heel. I imagine the same thing could happen if I was running in them.
Overall I like these socks a lot - but I almost have to consider them as "wear a few times and dispose", which is a shame because they are not exactly cheap!