For me personally:
I think the planar bass with virtually no roll-off, (emphasized with a small EQ in the sub-bass if desired), combined with the rather intimate in your face - albeit small soundstage, the rather forward mids and the peak in the higher mids/lower highs, which gives electronic snaps and crackles their aggressive whip-like effect (which can be easily EQ'd down a bit for personal preference and comfort), the almost unnaturally quick decay and attack in general, makes a rather good combo for well-produced electronic music
(read along the lines of: trip-hop, idm, glitch, downtempo, psydub, ambient)
Even though the soundstage leaves something or even quite a lot to be desired, the instrument separation and clarity is quite good - you'll probably hear a lot more information regarding especially the timbre in the bass after a dynamic driver - which can be a problem, if the production/recording just hasn't addressed/thought of these issues.
I think these cans really bring out the best, and the worst in a recording, even at this price point - which is rather awesome.
The above-mentioned qualities have and can cause a bit of a tin-like effect to the sq, along/coming from the source - not to mention the stock grill vibrations/glare, but I guess I won't go there.
Additionally, I think these characteristics can be the reason why HE-400i has been called a "warm" can, and a "bright" one in the thread over the years
Those are the thoughts on electronic music only, I've actually started to listen quite a lot more of various electronic genres due to these cans.
And other genres of music, a whole different story.
And as always, YMMV.