I got these as a backup to my RE-262.  I wanted something I could run without an amp.  The 400 provides that refined HiFiMan sound I have come to admire.  The 400 absolutely squares up with my 262 when amped, but I really got them so I could run lighter (just a dap), they totally impress in this regard.
I think I may be the only one that really liked the twist-to-lock fit of the 262, they just work perfectly for me.  Moving around I loose the seal on the 400 a bit too easily, but there was a generous helping of tips to play with including comply which I have never tried.  I am quickly becoming a comply convert.
 
Phone + amp + dap too much for your pockets?  Need to simplify?  this may be the solution.  It works for me, I don't mind leaving the amp behind any more.  The RE-400 fills the void until I can plug into a full size rig. 
For a Benjamin the cost to quality balance on these is stone solid.  They look great, sound great, and anything can play them.  Build quality looks like a nice step up from previous generations of HiFiMan portable gear too.
 
A word of caution:
While any flea-powered source can drive these, this is a high quality IEM, they will reveal how horrible music sounds on your phone...regardless of encoding, regardless of make.  You can listen to your phone with the RE-400, but to hear them you still need a decent dap (sansa clip is disposable at $40 and still sounds about 1000x better than your phone...ok, maybe 100x better than the htc one).

Buy it, mod it, slip on a pair of alpha dogs, smile.
Not worth owning if you don't mod.  I got maybe a half hour on them unmodded just to satisfy my curiosity.  I own better cans that cost more and less, so pretty worthless to me until modded.
Ratings are based on modded cans including comfort strap and alpha pads (comfort and sound).
You will forget they are ugly as soon as you put them on your head.

Mike and Jason have now successfully parted me with a tidy sum, but when I listen to their gear I smile.  I have an old pair of Grado SR-225's that I now realize I have never really heard.   I had already owned the Bifrost for my stereo but I had been driving the Grados with what was to me a nice sounding chip amp but I got tired of feeding the hungry little monster pairs of 9v batteries.

I could wax prophetic for a few paragraphs but I will spare you the agony.  The sound can be summed up as that sweet even order harmonic clarity characteristic of single-ended circuit designs.  For me there is nothing that comes close to the purity and musicality of a single-ended class A amp. 

The first couple times I put the phones on I had that sort of initial adjustment that I sometimtes get listening to my SET amp; it's sort of a feeling of being mentally jarred by the clarity of the music.  The sound is neutral and well balanced, has the ability to handle big dynamic swings and the separation of instrumentation is excellent.  It produces more of a head-stage than a sound-stage but maybe that is the Grados.  Noise floor is non-existent, even on the sensitive SR225's.  Oh and yeah...for the record, no need for A/B listening; the Asgard totally smokes the chip amp.  I was expecting subtle improvement but the two are in totally different orbits.

In sum if you are ready to step into the world of dedicated headphone amps, I find it hard to believe anybody with a decent set of cans would be disappointed with Asgard.  Given the reasonable price it can't be ignored.  Oh, just in case it hasn't been mentioned in any of the 1,600 posts in the appreciation thread it runs hot - really really hot (and stinks a bit when new) but hot-running amps usually sound good.  I do beg to differ with the instruction manual, it is an excellent coffee warmer...warranty be damned.

Here is how I listen:
#! Linux running MPD > USB > Bifrost > Asgard > SR225