So I finally got my pair of IM50's today, bought from Yodobashi Camera by a friend. They've got only about 10 minutes of play time on them, but here are my thoughts so far:
1. Holy crap that's a lot of bass for an IEM. I thought the CKN70s were fairly bassy phones, but these are nuts. I don't think I could stand to listen to the IM70s if they have more bass than the 50s.
2. The bass is pretty deep, but so far it's pretty bloated. I'm hoping this is something that not only tames down but also tightens up over time.
3. I really like the mids. They're forward but nicely defined and don't get in your face.
4. The highs are also quite nice. Not sibilant or harsh, but still crisp. Trouble is, it's hard to really tell how decent the highs are because all I hear is bass. But if I throw on an EQ curve that rolls of the low end, the highs are definitely there.
5. Build construction is great, they fit very comfortably, and the memory wire works very well for me. The cord is decent too, but about 2 feet too short for my taste.
6. For $60 or so, even if the sound doesn't change during break-in, you'd be hard-pressed to find a better value.
I'm not the biggest believer in burn-in, but for A-T's headphones it does seem to make a difference. I'll throw these on the iPod burn-in rig for a couple of days and see if that low end gets tamed a bit.