part of the issue with Elear reviews is that it's kind of an oddball headphone in what is should be competing against. It's basically a maxed out "easy listener" headphone. So, when a review compares it against things like the HD650, Nighthawk, LCD2 and other premium listening headphones, it comes out very positively, as it's just simply better than those headphones in most cases. However, price range, it squares more readily with things like the HD800, HiFiMan Edition X, and LCD3. People who have those headphones are usually transparency and resolution maximalists, and don't see what the big fuss is about the Elear. To them, it's too smooth and lacks micro detail.
Another complicating issue is that the Elear HATES anything with output impedance above 1 ohm. It can sound very dull. Doug from ECP mentioned this might be due to it being designed more like a speaker than a typical dynamic headphone (which is basically an inversed microphone more than it's a speaker). Because it's designed more like a speaker, damping factor is even MORE important than it is on most headphones. So some of those reviewers may have been using their super high end OTL amps, that have output impedances in the 10-30 ohm range, and that would be an awful match.
If you like smoothness and richness in the midrange, lack of harshness in the highs, and you like punch, and you like a nice bass kick that isn't overwhelming, but don't care about the last bit of transparency and treble extension, the Elear is for you. It is the absolute very best at what it does. Is it an upgrade on the HE400i? For most people, probably. They are slightly different flavors. But the Elear is as transparent as the HE400i, has roughly the same soundstage, is much smoother, has less offensive treble, is more dynamic (which is saying something because the HE400i is very dynamic), is drastically more punchy. It's a little less up front than the HE400i, but is more up front than the HD650.
It's certainly not a side grade, the Elear is just a better headphone, but there may be some people who would take the HE400i over the Elear, if they really love the HE400i's treble attack.
Comfort is also another issue that could go either way. The Elear is more heavy, but I think also better designed ergonomically. I found them both extremely comfortable, but I could see some people preferring one or the other.
Looks wise is a matter of taste, but you'd have to have some very weird theories of aesthetics to prefer the looks of the HE400i to the Elear. The Elear looks every bit of its place of a luxury headphone. I don't mid the HE400i, but it absolutely looks like it was designed by a bunch of engineers.