Just got them.
Fantastic clarity and imaging. Wow. The "in-ear HD800" people weren't kidding. The closest I've heard to the HD800 in clarity so far. Separation, control, everything "technical" about these is remarkably well refined. Midrange tone is dead-on accurate and extremely smooth. Bass drops off sooner than I'd like, but not to the point where it's problematic.
Same balance issue I had with the K1000 and Q701 in the upper ranges. I don't know what it is, but something sounds "off" -- not too bright, the opposite. Like the upper ranges aren't fleshed out properly. It seems to make things sound dead, uninteresting, non-engaging. In order of decreasing prominence, it goes K1000, Q701, ER-4S. It's only minor with the ER-4, to the point where I'm sure I could get over it, but the K1000 and Q701 were another story. Neither of those ever sounded right to me. It was like my music was trying to bore me to death, and in the case of the Qs, also stab me to death with sharp 2kHz distortion.
Compared to my Sansui SS-100, these are slightly more detailed and duller sounding. The Sansui's are more enjoyable and sound more natural to me. Conversely, the ER-4 slaughters pretty much all of the other headphones I have.
In terms of actual problematic issues, oh my god, **** these microphonics with a rake. The shirt clip helps a bit, but I am genuinely afraid of whats gonna happen when I try to use these while moving around in the soda cooler at work. Fit was also a bit annoying; there was a "sweet spot" I had to spend a bit of time trying to find, and I'm still not finding them particularly comfortable. But I was never an IEM guy.
This is with the S adapter. Without it, they sound pretty damn awful. Also, I'm using with the smallest triflange tips, the clear ones that come on the IEM out of the box. The other two pairs of triflanges are way too large and hurt my ears. The round/"glide" tips hurt too, and don't have enough bass. The foams are pretty comfortable but they're extremely bassy, not necessary a bad thing always... but still not accurate. The frost triflanges sound the best to me.
All of these impressions are from my home rig. I'm scared to plug these into a portable player... probably gonna sound dreadful.
TL;DR: Good. Cleanliness and clarity to rival high end headphones. Bass extension isn't the best, and the upper ranges sound a bit dull for my preferences. I like them, but they did not dethrone anything I currently own.
FWIW, they sound amazing using an Ipod Classic 5.5G, no amp. I commute with them up to 3 hours round trip every day (mostly sitting on a train, some walking). As long as I use the shirt clip and keep the cord inside my top layer so it's not swinging all over the place, the microphonics aren't an issue for me. I use the medium gray flange tips, after years using the smaller ones. The slightly larger ones took some getting used to but give me a better fit. Long live ER4s!