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
I've listenend to all the top offerings from Denon, Audeze, AKG, Hifiman, Ultrasone & Sennheiser and this is one of my personal favourites. It is a true reference class 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 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 sound quality becomes no longer a factor of quality but rather personal taste. It really doesn't get much more "enjoyable" 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 "better" 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 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 imepedance 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 twice its price. A superb allrounder with a fun and energetic play-style, detailed but not too unforgiving - not to be overlooked!
Edit Feb 2016: I've decreased the rating down from 5 stars to 4.5 stars. The reason for this is two-fold:
1) The introduction of the HifiMan HE-400S
2) The introduction of the Philips Fidelio X2
If you like a neutral reference type sound that does well with all genres including classical consider the HE-400S, if you like your music with a lot of "weight" and bottom end grunt look at the Philips Fidelio X2. I would personally not consider the X2 a reference type headphone, but it's still an excellent entry point into high-end audio for those that like a meatier fuller sound. My own preference is towards the more neutral sounding HE-400S, HD650 and HD600. I would personally probably still pick a well amplified HD600 or HD650 over the HE400S, but dollar for dollar the value of both the HE-400S and X2 are outstanding.