Pros: Richly detailed, supremely textured, natural sound that is smooth, effortless and relaxing to listen to. A forgiving allrounder with superb comfort.
Cons: Open headphone that requires good source and amplification to perform its best
I've listenend to all the top offerings from Denon, Beyer, Audeze, AKG, Hifiman, Ultrasone & Sennheiser and this is one of my personal favourites. It is a true reference headphone that is very natural sounding, albeit leaning ever so slightly towards a darker tone. The sound is detailed, smooth and richly textured, especially drums and vocals come alive amazingly well. It has no problem spanning all the way from the deepest bass to the highest highs with supreme control and plenty of muscles to spare.
For the money I think this is a real audiophile bargain; the HD650 exists on that plane of ultimate performance where musical enjoyment no longer becomes a factor of sound quality, but rather personal taste. It really doesn't get much better than this no matter what you pay. The HD800 has better resolution, speed and spacial presentation - but can be a bit cold, analytical and unforgiving. The LCD-3 is arguably a higher-performing heaphone, with amazing resolution, extension and control, but in my humble opinion it can sound a little dry and laid-back compared to something like a HD650, Grado, Hifiman or D7000 which are more "fun" and energetic and with a more "forward" sound.
So in short - the HD650 I think is truly great, not unbeatable in specific genres but an amazing allrounder. It should however be said that it only truly begins to show what it can do with top notch source and amplification - I wouldn't recommend driving this beauty with anything like a phone or Mp3 player due to its rather high impedance and revealing nature.
Although "revaling" is also a relative term since they are infinitely more forgiving towards bad recodings than a HD800 or LCD-2/LCD-3. It should also be said that I do think they exhibit a slight veil (this eternal debate) but ONLY at low listening levels - so that those who find themselves always listening at very low volumes might want to also consider something along the lines of a grado reference headphone which are known for their "intimacy".
All in all the HD650 is a superb headphone that comfortably matches, or beats, competitors at more than three times its price (not joking). A superb allrounder with a fun and energetic play-style, detailed but not too unforgiving - not to be overlooked! Seriously.
Edit Sep 2016: I've increased these back to 5 stars again. After changing DAC and some other system tweaks I managed to elevate my HD650's to a yet another plane of performance. They just scale like crazy and completely unlike any other headphone in this price range with the exception of the HD600. Sorry for the confusion.