I've got a good amount of time on them (some "unattended" burn in as well), my thoughts so far, and comparisons to my Beyerdynamic DT880, Premium, 250ohm.
Packaging, build, etc., as expected, just terrific. Lightweight, decently comfortable, a little heavy clamping, I adjusted the metal bands a bit, feels pretty good.
Have listened to these artists so far:
Massive Attack
Pixies
Radiohead
Metallica
Tool
Yo La Tengo
Pink Floyd
Peter Gabriel
David Bowie
Beck
R.E.M.
Portishead
Tori Amos
So mostly a mix of alt/rock/prog/electronic-trip-hop, but lots of instrument, vocal variation, across these sort of related genres, all from CDs on the Teac P650.
I like them quite a bit, awesome sound stage, warm but not sloppy, nice lower end bass, a touch syrupy, but still plenty clean, fantastic mids (and a little forward), highs are warm, a little "laid back", detailed but not overly energetic. Vs. the DT880s, well, they're very different, the highs on the DT880s kind of sparkle, but depending on your taste, can be too bright, the mid-bass is fast and punchy, but the lower end bass isn't as present but has more slam, and they're definitely more V shaped, the earpads on the 880s are also a bit more comfy, I might look into aftermarket pads for the 6XXs.
In the under $500 category they're very competitive as the HD650, as the Massdrop 6xx (basically a 650) at $199 (delivered in the US), they're nearly unbeatable.
There's another drop on these currently active:
https://www.massdrop.com/buy/massdrop-sennheiser-hd6xx
For a new desk setup, I'm thinking about a Schiit Vali 2 (with a tube swap), and a Schiit SYS to switch sources between the CD player and my Mac - the DAC in the Teac is pretty awesome, but I may still add something like a Modi 2 Multibit for the Mac (and/or for the Teac, after I get some time with it). Then I plan on using the pre-amp out of the Vali 2 to a set of KRK RP4G3 powered speakers for when I want to skip the HPs.