Pros: Bass/low end, soundstage, comfort, isolation
Cons: Understated treble, slightly muddy midrange, very difficult to get in your ears
These phones have one of the best low-ends out there in the IEM category. The bass is huge, clear, and REAL; I would not describe the sound as bloated or artificially "thumpy" as some other bass-heavy phones tend to be. They also have the darkest sound of any phones I've ever heard, which works well for some music and largely depends on your personal preferences. The warm sound is excellent for jazz/blues and are my go-to phones for recordings which tend to sound sibilant. The soundstage is also quite good for IEM's. They do NOT have to be amped to produce quality sound, although I don't buy the Future Sonics claim that these perform exceptionally well at low volume levels because I frequently find myself wanting to turn up the volume to make the sound more dynamic. The treble is clear and sounds quite nice, although it is understated. I sometimes find the midrange on these phones to lack clarity. These phones are NOT analytical, but as such can be listened to for hours without substantial fatigue.
Getting these fit into your ears is simply a pain. They come with a variety of sizes of double-flanged silicone tips and foams, but they all are a chore to use. The sound depends tremendously on achieving a good seal and getting the phones deep into your ears. Other tips can improve this; I use Alpine double-flange silicon tips which improves the difficulties with fit and helps bring out a little more treble. Despite the difficulties with fit, once these phones are in your ears and correctly adjusted, they are quite comfortable. Isolation is very good.
While some people do not like the design on these phones and consider them dated, I actually like how they look and find the shape to enhance their comfort. They need to be worn over-the-ear to reduce cable noise. I originally had Atrio M8 v1's which had "memory wires" coming out of the phones which were discontinued in the current model Atrios, which I believe was a good call. The build quality on these phones is better than average but nothing spectacular.
All things considered, these are great IEM's for someone looking for a warm, friendly listening experience. They are getting to be a bit expensive given the lower priced Hippo VB which have been described as having a similar sound as well as other high-quality IEM's (I'm looking at you, HJE900's!) at less than half the cost of these, but I enjoy mine. The newer MG7 drivers coming soon for the Atrios allegedly improve the treble and mids of these phones, which I am very interested in hearing. If Future Sonics claims are true, they could have a serious winner on their hands!
EDIT: I discovered that the small double flanges work better for me in terms of both fit and sound quality, despite the fact that I use medium flanges/foams/tip/etc on every other earphone I've ever tried. So, the fit issues I discussed above...it's still not super easy to get them in your ears, but it's better for me than before. Also, the treble isn't quite as recessed, and the mids are clearer, though I still would say my above description holds true. But the bass...oh, the wonderful bass...I dare you to try and find phones with a better low end then these at < $500.