Got mine today (thank you China next-day bike delivery) and have been listening for a few hours.
My early impressions below (using large stock tips worn straight down).
These LOOK very good. The materials used don't actually feel that high quality, though they are very well designed. The packaging is great (and ya it smells like chocolate which is freaky). I love how Xiaomi not only copies everything Apple but even make their accessories Apple-supported, so the Apple-style remote on the cable is a plus.
Comfort & Fit
The Pistons don't seem to be designed for very deep insertion, as the housing scrapes up against your ear when you press it in. They are very light though and with the stock tips have rested in my ears for last few hours with no problems (will try Monster gels and comply tips later).
Good for a 100RMB IEM ($16). Really good for that price. In general... not great by any means.
I remember a time not long ago when it was impossible to find decent bass on cheap IEMs. The quantity here is good, especially compared to Apple's IEMs which are an order of magnitude more expensive. Even without a good seal you get good bass quantity which surprised me a lot.
These are bass-heavy earphones. Very full-bodied. Deadmau5 was awesome here. Pretty Lights was so fun.
Unfortunately, there is no real sub-bass to speak of (missing rumble), and the mid-bass– though a nice thick/smooth sound– is not particularly "fast" or accurate. The songs I always use to test bass on IEMs are "Hip-Hop" by Dead Prez and the Zeds Dead remix of "Paradise Circus" by Massive Attack. The bass definitely comes out (way better than any other cheapo IEM I have tried), but comparing to IEMs in $100+ range? They lose clearly.
Mids / Treble
The upper bass comes into the mids a little too much. The mids are recessed to begin with I think, and also a bit thin and lacking in detail. The mids are not bad at all, they just don't "sing" as well as other (much more expensive) IEMs. This is the case literally on several tracks I listened to where the vocals really seemed overwhelmed by the warm signature. It doesn't happen on all songs, and I found it funny that a few hip-hop artists I always thought were too "in your face" were more tolerable on these phones :) – I'm talking about people like Common Market and Immortal Technique.
In general I found vocals and other mids just a bit too recessed and veiled, leading to me missing some intricacies that make vocals powerful. "Little Lion Man" by Mumford and Sons, for example, is usually an emotionally gripping song for me but with the Pistons it felt more like a sick beat instead of the awesome lyrical journey it usually is.
Like the mids, the treble is not bad at all, but lacking in detail. Treble also does not extend too far, though in my opinion this is not a huge problem. To me the bigger problem (and I say "problem" here only in relation to IEMs that are not $16) is detail is missing. Again, female singers that are usually heart-melting for me (Kinna Grannis, Priscilla Ahn, Lauryn Hill) did not capture me or take me on a ride.
Way better than other cheapo IEMs I've tried, though not "spacious". I did listen to the virtual barbershop video on youtube today and it owned me with these on :)
Despite not easily being able to get a good seal, several people at work tried to talk to me today and I had absolutely no idea they were there. Volume was not even high, so I guess isolation is good (and I'm a horrible manager!)
Horrible when worn straight down, not a problem when worn over the ear.
So in general, these earphones– which are basically free– are incredibly high value in all aspects. I'm not sure, however, that they measure up to the immense hype I've been reading in this thread! I very much doubt they match the Dunu 1ks for instance (though I've never tried), and I will make a comparison when I go and demo (aka buy) the Dunu 2ks tomorrow :)
I agree with you on soundstage. I personally think the soundstage is compatible to that of the older model soundmagic pl30, with better clarity and bass.....but, of course less mids. I would suggest giving them at least 50 to 100 hours of burn-in time to get a better feel, and better results.
Also, ear tips do make a difference. Some reviewers say that the T400 comply foam tips makes a big difference and gives it great balance. I also suggested in this thread that the medium tips that comes inside your piston box labeled "M" might just be a better fit. And might even give you better controlled bass and better clarity.
None the less, it is a great buy! And I agree with you when you say "-are incredibly high value in all aspects".