So, I've had nearly a week with the headphones now, and wanted to give some updated impressions.
Firstly, these headphones are incredibly easy to drive. On my DX90, my normal listening volume with Momentum On-Ears is around 220 (goes up to 250). With these, it's around 200-210, which is similar to IEMs I use with it (all on high gain). Because of this, I can push them to levels where they distort (and also painful). This is good for anyone who wants to use their cans directly from a player without a more advanced amp stage.
Secondly, although they are 'Closed' and 'Open' headphones, they're really more just Open headphones with the ability to reduce the exterior sound level. They do leak sound with the covers on. It's not awful, but if you intend to only use these in public places, you would be better served with a set of closed back headphones. If I set the volume to normal listening levels, it hardly leaks any sound, but it also doesn't block out louder sounds.
It's very strange compared to some Open Headphones. If I put my HE-400 on, and put my hands over the grilles, it sounds awful. The Hifiman Edition S sound great both Open or Closed. And it changes the sound exactly as you'd expect. When the covers are on, the bass is more prominent and instruments get grouped closer together. When Open, the bass doesn't feel like it has as much impact, but the sounder has more air, and the soundstage is wider.
I really do like these headphones. The sound is around 90-95% of the HE-400 (original), but cheaper than the HE-400 originally cost, and with the added benefit of the ability to wear them out in public.
However, I do have one caveat. If you are ONLY searching for a pair of headphones that you will ONLY wear out in public, and will not give any home use, then these are not for you. The major benefit of these headphones is the versatility (and they sound really good too). That isn't to say that these are bad portable headphones - they are great - but if you are looking for portable headphones that block out as much noise as possible and leak no sound, these cannot accomplish both of those goals. The sound leakage isn't awful, but any design that allows a change between Open and Closed is inevitably going to lack the advantages of a true closed back headphone.
If you are looking for a set of headphones that you can wear in public and sound great, but when you return home you can unleash their other side, then I can't really think of many better options, and certainly none at the same price point.