Well after one week of ownership, I am quite pleased. I have been playing them all all day while I am at work beginning the break-in process;but between listening and playing they probably still have only about 60-70 hour of use. I am guessing the headphones should be "broken in" in the 200-300 period. My pre-purchase rig was a Grace Design m903 and AKG K702s so I did do a lot of A/B ing between them and the HD800s. BTW, serial numbers are now up in the 178xx range for those of you you track such esoterica.
I think that they are as many of you know fantastic headphones for someone seeking a magnifying a sonic magnifying glass into the recording studio or recorded space. If find them very much in the school of the AKG K702s which at 1/5th the HD800s price are truly remarkable headphones. I find the Sennheiser's are buying me more bass information, detail information; both sonically and spatially and they also are more comfortable to wear. On a qualitative scale, I would say the extra cost sonically is getting you that last 5-10% of information sonically and spatially that is not presented by the AKGs. Such is the cost/benefit curve once you pass the asymptote which I would say the AKG's lie on. The last bit of information always costs you a disproportionate sum (diminishing returns).
I will be curious to see what a different amplifier will be able to do beyond the Grace Design m903 with them. I suspect the difference is similar to what I found with the AKGs. I might find that last 5-10%. I am still on the fence to go with a nice tube amp like a DNA Stratus 2A3 or a balanced SS amp like a AMG Audio Sigma/Beta 22. I think either might be a step up from my m903. Unfortunately, neither of these amps are readily available for auditioning like headphones (I compared the HD800s to the LCD-2s with library loaners). Both choices have 4-pin XLR balanced outs, so a custom cable will be in order as well.