The HiFIMAN HE-400 is HiFIMAN's current best seller. After spending some time in it's company, it was a no brainer as to why. It is one the only conventionally styled headphone I am aware of, that gives you planar technology without baggage. By that, I mean you do not need a dedicated amp, or any other headphone accoutrement to get planar performance from your computer or hand-held device. At $400 (or any price), it's alone in my estimation both in price, sensitivity and the associated tech.
The HE-400 will not leave any dyed in the wool audiophile, feeling in the least compromised and in fact they'll be noticeably satisfied, and feeling rather intelligent at how good things can be without cracking the piggy bank.
That said, the glory of the HE-400 is in sharing; for the uninitiated, it is an instant evidentiary experience of high resolution. I've had responses from "OMG, these are fantastic!" to "Thanks for nothing, now I know just how crappy my headphones are..." And it's scalable, still, without any other purchase, (aside from the computer or hand-held i-device or pda we assume you already possess) you're in hifi. If you decide to go to a more resolute hand held; like the HiFiMAN HM-901 or the Astell and Kern AK100, the HE-400 has no issue showing you just how much more it can do. The same can be said for pairing it with some rather lofty headphone amps. Really it's not a sensitivity issue so much as supplying it with better current and signal.
I also did some experimenting with the HE-400 while commuting. I thought it might be helpful to those on the go, or residing in a city. The subway in Boston (commonly referred to as the T), is the oldest in the country. To those of us here in the heart of Bean Town/Cambridge, we're acclimated to what others might relate to as, a kind of fairground rickety roller coaster, without the great ups and downs. Compared to other trains I've experienced, it's like commuting in a turn of the century ice cream truck.*) The open ended nature of the HE-400 did not fare well in this surrounding with my iphone working as both amp and source. That said, it worked exceptionally well when I headed North to Maine on Amtrack or when on a Continental Bus, and even on the city bus. So, it's not noise canceling and you can't expect it to over-come areas wherein the surrounding volume is high. All those caveats disappear, if you're using a small headphone amp, or even one of the more powerful hand helds like the HM-901 or the AK100.
My suggestion, is that if your use would be one of commuting, to try it first, ymmv. Surprisingly, others around me didn't complain about the noise coming from my headphones. It likely didn't compete with other ambient happenings. Finally a headphone that brings planar resolution without any additional boxes for less than it would cost to buy a decent set of dynamic headphones and a decent little headphone amp. These things score a 99 when it comes to showing non-philes the point of this hobby and why it could apply to anyone who loves to listen to music.
Kudos Fang Bian!!!
*The issue of the MBTA and their transit system, and many US systems is one of small thinking. For decades, the MBTA has run at a fiscal deficit. It does so because the schedules are often redundant, the service unpredictable vs. the published schedules, and the trains and buses themselves of a vintage nature. Each year there is talk of cutting their budget, when what they should consider is doubling it, under proper supervision. If they made a system that worked, the number of constituents who would take advantage of the service would easily triple.
As a case and point, I look to Jaime Lerner of Curitiba, Brazil, who took a population of 300,000 and gave it an infrastructure to support 3,000,000 people by getting cars out of the center of the urban environment, and putting in a superb transit system. Everyone from retailers to real estate professionals, to the common man saw huge gains, in lifestyle and dividends. Jaime is in charge of revamping the RIo transit system prior to the next World Cup. The plan calls for a goal, wherein there will be no more than a 60 second wait for a train in the city. Real distribution. For those of you with a city planning bug, you may enjoy this TED Talk.