Just took delivery of my HE-400i yesterday, and got a couple of hours of listening in last night.
Below are some first impressions. For a bit of context, I'm somebody who is coming from the Sennheiser HD598 and the AKG Q701. Most of my listening was done through the NuForce UDH-100 DAC/amp combo.
First - The headphone is comfortable, but the clamping force is kind of tight out of the box. That was solved by stretching the headband a bit and leaving it in that position for some hours.
Second - These are not bass heavy cans, like others have said in this thread. Rather, the bass is what I would call accurate - if the song calls for lots of bass, the 400i will deliver lots of bass. If the song is not bass heavy, then the 400i will accordingly not provide that much bass. The 400i actually made me realize how some songs that I thought before had a lot of bass actually didn't have that much. Bass is also amazingly detailed and textured.
Third - I think people have overstated the lack of airiness and the closed-can feeling of the 400i. I was actually pleasantly surprised by how well the 400i handled classical and orchestral music. Sure, I think it does lack that tiny bit of airiness compared to something like the HD598 and the Q701, but I would say that the 400i has about 95% of the airiness that those two headphones have. But the 400i is in another class altogether for transparency. As for soundstage, it's actually decently big. Good depth, and actually surprisingly good width (better than what I remember when I listened to the prototype version). If I had to compare the soundstage width between the 400i and the HD598/Q701 (because the latter two have what I would consider about the same soundstage sizes), then I would say that the 400i has about 85% of the width of the latter two headphones. And I haven't done the grill mod on my 400i yet - it can only get better from here.
Fourth - Although these are fairly forgiving headphones, they still will make very apparent the differences in the quality of mixing and mastering between different songs. Basically, while some poorly mastered songs will sound "good" due to the somewhat smoothed (but still highly extended and detailed) treble, songs that are excellent recorded and mastered will sound "excellent" and will be obvious next to the more poorly-mastered music.
Fifth -This thing is actually sensitive enough to be decently driven by smartphones. I listened to the 400i for a bit using my Sony Xperia T as a source. The Xperia T can't actually drive headphones to a very loud volume, so I had to crank up the volume on that to around 80-85% to get to a comfortable listening volume, but the 400i still sounded very good out of that. (For comparison, to get to approximately the same listening level on the HD598, I have to crank my phone's volume to about 70-75%). On another note, the 400i seems to scale well with your gear.
Sixth - Tonality and transparency of the 400i is excellent. Everything sounds just right and extremely realistic with the 400i, especially guitars, pianos, and vocals. The Q701 lacks this transparency in its mids and highs, and the HD 598 while sounding tonally accurate like the 400i has this raspiness/harshness that is totally not present on the HFM headphone. Perhaps this is what people call "graininess"?
Overall, so far I'm extremely impressed with the headphone, and I feel it is worth every penny of the $500 I paid for it.
I'm planning to write up a review later after I do more critical listening and let the headphone burn in for the amount of time that HiFiMAN suggests (though I am somewhat skeptical about needing to burn it in over 100 hours). If anybody wants me to put in something for the review that other reviewers haven't done so far (though I'm pretty sure they've mostly covered everything), just let me know in this thread and I'll see what I can do.
* EDIT: Minor addition to the fifth point, to give a comparison of how sensitive the HE-400i is compared to the HD598.