I still need to long-term test them, but am very impressed so far! They are very comfortable and light-weight, absolutely no bulk at all, the compression level feels just fine. You get more stability without losing any range of motion, and I'm pretty sure they are going to be awesome especially for pointe work (thank you, hypermobility). I'm a street shoe size 8.5, and an M fits perfectly. Will definitely incorporate them into my training routine, especially because they disappear beautifully into the ballet tights and shoes.
I really wanted to love these socks. But I wore them around the house briefly and found that they irritated my Achilles tendons. They were red after I took the socks off. It could be just me, as I have heels that stick out so my Achilles tendons are more exposed than for people with heels more in line with their ankles. I got a small and I wear a size 7 street shoe. The socks seem to fit fine otherwise.