So...as always YMMV.
Just my short impressions....
I took a listen to the 820 today. It was through the Sennheiser 820 amp. Let's get this out of the way first - I did hear a slightly nasally/boxy midrange. It was on all tracks and the 2 workers in the store heard it too.
Now if you're going to charge this amount of money for a headphone, it's got to be damn near perfect, or with no fatal flaws. This nasal sound, to me, is a fatal flaw at this price point. The other bad points - the sound is pretty unexciting. Not really suited for dance and electronic styles, which call for punchy bass, or rock to a lesser extent. Highs seem rolled off, especially compared to the other 800's.
The good - jazz and classical sound awesome on this. There is refinement and smoothness up the ying yang. Resonances seem to have been neutralised, except for that boxy midrange. Soundstage is as wide as I've heard on a closed can. Imaging is impeccable. Just pinpoint accurate.
Now for about a third of the price you can get the Aeon Flows, which achieve similar levels of smoothness and refinement (probably missing 5-10% of the 820). However, IMO, the AF is much much more exciting to listen to with a wider range of genres. There is also the possibility to tune the sound to suit your preference easily with the tuning filters.
I'm not going to labour my point. I think it's a bit rude to trash the subject of a thread too much in it's own thread. My intention is not to sour the purchase for any owners. If you don't hear the flaws or share my opinion, then that's cool.
At the end of the day the price is too rich for me, and that's a moot point because I don't like the overall sound signature anyway.