I have the Ether/Ether C on loan and I did some listening last night vs. my HD800S.
TLDR Version
The Ether is legit. I like it better than any Audeze I've heard. It won't be supplanting the 800S in my collection, but this is a legit good headphone. Frequency balance issues and price are its biggest weaknesses. Bass and comfort are its biggest strengths.
Long Version
For reference, my chain: iPad -> Wyrd -> Yggy -> XP-10 -> DSHA-4.
Pros:
Ether bass is fantastic. Waaaaay better than HD800S bass. Going back and forth you can really hear the distortion on the 800S. The Ether feels like it is clear an octave lower than the 800S. The bass quality and amount feels just about perfect with the Ether. The Audeze cans I've heard hit a little harder, but the Ether is totally fine.
Going back to the 800S makes you sad how mediocre the bass is. It isn't bad, but the Ether shows how much better and clearer bass can sound. The 800S really falls apart in the bottom octave.
Comfort is surprisingly good. While the headband felt kind of finicky at first, the comfort once these are on my head is very good. I like the 800S fit and feel better, but the Ether is well above average when it comes to comfort.
Soundstage is better than what I've heard from Audeze.
Distortion is low, and you can totally hear it. These are very clear sounding headphones and they handle power pretty well.
Cons:
The Ether really should be $995, not $1499. If these were under a grand, I'd think about buying a pair.
Cable length is too short. I guess its the DUM balanced cable that came with the loaners. Not all of us listen at our desks. The 800S cable is 3 meters and the DUM cable really should be that long as well.
The Ether sounds thin to me. The 800S treble and mids are better balanced to my ears. The Ether doesn't sound bright, but the balance throughout the range just isn't there the way it is with the 800S. This is the biggest fault of the Ether, IMO, and its what will keep me from buying one.
If Dan an co can fix the treble and mids balance in whatever they come up with to succeed the Ether then they might have a real winner.
Also, Sennheiser, give me a damn TOTL planar magnetic already!
The 800S soundstage is much better. The Ether doesn't stage poorly, but it isn't in the same league as the 800S. Going from the 800S to the Ether I missed the staging right away.