Really, what can be said about these that haven't been said? They're basically Sennheiser HD 650 headphones at a fraction of the price. Absolutely incredible in the mid region, good treble (clear, but not the most detailed), and good bass (not the most extended, nor are these going to rock your world with the bass impact, but the quality of bass is just fantastic; buttery smooth and chocolatey). Soundstage is a bit small, but it is reasonably accurate and acceptable overall; no "blob of sound left, blob front, etc," but you definitely won't feel like you're sitting in front of the band.
What these headphones excel at is classical music, particularly strings, piano, and in vocals; if you're a big fan of Celine Dion/Andrea Bocelli-type music these are just sublime. Orchestral pieces are also just mind-blowingly good, and I thoroughly enjoyed "All of Me" by The Piano Guys.
Some rock also plays very well, such as "Thought Contagion" by Muse. While the guitars in rock do tend to sound a little warm and laid-back, it really works well in some songs, and doesn't work as well for certain others; I wouldn't listen to speed or heavy metal with these. You'll want something more dynamic with a punchier sound for those tracks (and for the current Top 100 for that matter), as these are pretty laid-back in their presentation of sound.
I tried plugging it directly into my phone for giggles and to my surprise, my Galaxy S8 actually drove these reasonably well. Although it should go without saying that an amp goes a long way with these cans. There is a noticeable difference going from the phone to my Schiit Magni 3 to the Woo Audio WA7.
This brings me nicely to the topic of scaling; the Sennheiser HD600/650 is known for scaling incredibly well. If you like the sound of these headphones, you'll definitely want to keep these around because they grow with your equipment upgrades, which is the reason why lots of audiophiles have held onto their HD 600/650.
I do have a larger (but not huge) head and the clamp force is a little uncomfortable in the beginning but I quickly adjusted. Ear pads are of good quality velour and are comfortable.
Overall, just absolutely fantastic headphones. If the fact that they're still produced and talked about despite being released in 2003 doesn't say anything about their quality and sound, I don't know what else it would take to convince you.