These are, imo, the best Sennheiser headphones. Among the many headphones I've tried, the only ones that feel really neutral and natural. A perfectly balanced sound, smooth yet very detailed. Bass has very good extension  and awesome overall tone and impact. Mids are magical, dead neutral and flat and you really feel this on voices and strings. Highs are very extended and smooth and there's plenty of detail. An awesome allrounder. Best value headphones out there, with X2 as a close second. Sounstage is very good but not on par in tridimensionality with that of hps such as HD800 and X2.
Build quality is great, very sturdy but light, and they are very nice looking. Cable is good but improvable, HD650's cable is better. Comfort is really great, they disappear after a while and theyb stay perfectly fit even while moving your head. Pads are very comfortable.
I find HD600s better than HD650s, much more natural mids and overall sound , they're more involving with better bass (hd650s' bass has too much mid-bass and less impact). HD650s are still very good though.
X2s have a more tridimensional sound with more bass but lack some naturalness and smoothness in mids/highs, there is a notable peak up there. HD800s have better soundstage but everything else is worse because of another peak in treble and mids are a little too recessed.
Overall HD600s are to me the best hps out there because they are very good at everything and have no big weaknesses, altogh newer hps may be better in specific fields. They are the only ones with a totally neutral, smooth and balanced sound. Very very pleasing, I consider them my reference headphones to compare others to. Definitely a MUST-HAVE.

I own the desktop version of the O2. What to say? It is just perfect in every sense. No coloration , no noise, great power, best tecnicalities out there. BEST VALUE EVER. Endgame amp.

Very, very, very good headphones, Kudos to Philips for making such a stunning performer.
Neutral headphones, they still mantain a very high fun factor and high trasparency. Very detailed yet very musical too. A true masterpiece. Bass is extended, creamy, textured, detailed and with good presence. Mids are perfection. Highs are smooth yet airy and sparkly, holographical. Not laid back in any way, yet smooth. Very very easy to drive, even from a smartphone. Very high trasparency and wide soundstage. They only lack the ultra uber high end tecnicalities of a 30.000$ system, but not even HD800s have that. Still burning in probably, here are some impressions on some songs.
Check the end of the review for burn in impressions.
Animusic: 

Acustic Courves 
Oh my god. This is wonderful. Incredible detail, with a supremely organic and layered sound. Pluck of guitars is very satisfying and with excellent texturing. Drums are excellent, with great body and presence. Very realistic.
9.5/10
The Harvester
The strings. Ohhhh. How good, so organic, yet realistic. Veeeeery very good. And then the electric guitar. It's just perfection. Not  too intimate, not too spread, with right amount of body. X2 has a very high ability to mantain high contrast and sound still fun and smooth.
10/10
Pipe Dreams
Again very very good contrast, organic yet airy sound (!?!?!). Very very enjoyable and neutral. Very good soundstage. Instruments are all around, you feel in the room.
9.8/10
 
I Want To Spend My Lifetime Loving You - Tina Arena & Marc Anthony

Vocals are sooo good. Very good extension and transparency. Very very enjoyable. Addicting. Exceptional naturalness and wide, immersive soundstage. Very controlled, great body.
10/10
 
I will always love you - Dolly Parton & Vince Gill

Again, vocals are just exceptional. Stunning. I don't know what to say, I would just repeat myself. This is incredible, how balanced the sound is, yet how enjoyable.
9.8/10
 
Digital Love - Daft Punk

 
Bass is really great. Very immersive. Vocals, as always, just perfect.I'm stunned by the ability of these headphones to never sound congested. Very good detail. BAss is very "open", almost as if in a real room. Definitely not boomy, very very good control and transparency. And again, that feeling of enjoyment. You forget the headphones.
9.5/10
One Day (Vandaag) - Bakermat

Exceptional realism. Pianos have great body and realism. Bass slam is fenomenal. Very good Prat and impact. Perfect vocal rapresentation. Very good soundstage. Right amount of sparkle.
9.5/10
 
What I Might Do - Ben Pearce

Stunning bass performance, vocal perfection, very detailed yet involving. Bass is almost creamy.
10/10
 
Bust A Move - Infected Mushroom

 GREAT! Really great! Very airy and detailed. Bass is again present and impactful, creamy. Cymbals are higly realistic. I hate repeating myself, but again, vocal perfection. Incredible soundstage. Very airy. Wonderful imaging abilities.
10/10
Savant - Nazareth

 
Very high realism and contrast. High detail retrieval, great soundstage. Very good impact and PRAT. Very credible. Still pretty smooth anyway.
9.5/10
 
Savant - Wild Ganja

Again a stunning performance, perfect balance, extremely wide soundstage. Bass has very good punch and presence, mids are perfect, highs detailed and smooth but not laid back.
10/10
 
Moonlight Sonata - Beethoven

 
Exceptional. Stunning. Lovely. You feel the notes on the piano as if they were real. Black background. Very immersive.
10/10
 I will add more songs and impressions as burn in goes on.
These are very very very good allrounders anyway.
 
Having owned HD600s, wich i loved very much but ended up selling after getting some HD800s, I have to say these are clearly better. Yes, better than HD600s and near HD800s. I'm the one who spent 1200 dollars for HD800s anyway
Vs HD650s: X2 has much better soundstage,sound is more realistic; definitely more enjoyable sound (especially bass is more extended, textured and punchy, one of the weaknesses of HD650 IMO), highs are more airy, much more holographical sounding but still lightly warm and smooth sound. Mids on X2 are more neutral( I always found HD650 to be emphatised at around 500Hz) but equally natural. HD650s are a little more forgiving, treble is a little tilted down on them on some areas but more present higher up and i do not find it very entertaining. On X2s it is almost right, smooth, yet not laid back, definitely. There's sparkle. HD650 is near neutral, but there is a little veil, since it is a little warm but with not perfect bass extension and forward mid-bass. X2s are definitely the winner here.
 
VS HD800s:X2 has a little less wide sound,separation, detail and articulation on par. Still more enjoyable, but HD800's sound is a little more transparent (faster, more control, less distortion?) but not by much. still X2 has better depth, more credibility and a more balanced sound, in fact they are very good even without amp. HD800 has a more analytical sound (it is not bright, but bass is very fast and it presents detail in a forward sounding way). HD800 is tecnically better but X2 is the better headphone overall.
These are the most enjoyable headphones I have ever heard. Neutral but fun, with both a hint of warmth and of sparkle. Fenomenal. 
Comfort and build quality are very high. Pads are very comfy (no problems of ink leak), headband almost disappears. Comfort about on par with HD800s. Cable has much better quality than HD650's stock but seems less high end than HD800's.
Price/performance ratio is the best i have ever seen in audio. If these were selling for 900$, it would still be good value. Yes they are that good
I have had them for few days (almost two weeks), thus the sound may change ... will post if it happens.
 
Look is very good for the headphones by themselves, not so much on head because of the big headband system,looks a bit weird on head. Still very sexy though  
 
Overall thoughts:
 
Sound: 10-
 
   Bass presentation: 10
 
   Mids presentation: 10- (slightly behind HD600 wich is more natural)
 
   Highs presentation: 9 (a little artificial atm times, not the smoothest out there)
 
   Distortion measurements: 9+
 
   Space recreation and coherence: 9.7
   
   Fun factor: high
   
   Transparency: high
 
Comfort: 9.7
  
   Pads: 9.5
 
   Headband: 10
 
Look: 9
  
   Alone: 9.5
   
   On head: 8.5
 
Value: 10+
 
Overall: 9.5
 




 

 

 
 
 

 

P.S.: you want a more HD650ish sound? Try placing HD6xx foam, cut to fit, over the drivers(under the pads). Hd6xx sound with better extension and imaging.
 
Burn in impressions: sound is definitelt improving and I don't think it's placebo since with HD800 I heard no change whatsoever in sound . These are definitely changing. Mids are more forward now, the sound in general got more trasparent and a little more smooth. Maybe pads are changing with use? Dunno, anyway, for anyone buying them, wait at least two weeks of burn in until giving them a final judgement. Again, I'm shocked by the quality. Very very good Philips
 
Burn in 2: these keep improving it's crazy. Or I'm becoming crazy. Really, this cannot be true, for so little money. They are really really underestimated just because they are relatively cheap compared to the insane prices of hifi audio.
Revising votes as time goes. Sound gets a little more mellow

All in all I would say a pretty neutral sound with very good extension on either sides and very very good soundstage. Very trasparent. Sometimes still a bit hard on the ears and sibilant, definitely not as neutral as HD600. Pretty balanced sound signature but with a treble peak.
Sound is very good and clear but not good at all for the price. Bass has impact and extension, mids are very trasparent and realistic, higs have an annoying peak but otherwise are extended and balanced. Detail retrieval is very high but because of a peak in the treble it often feels artificial. Great impression of realism but sound is actually not very realistic. You always feel that peak. I preferred HD600s in the end, I found them to be superior in pretty much any area except soundstage width.
Comfort is great, they almost disappear and never touch the ears; build quality is very high too. Cable feels very very high quality. And they look really awesome.
 
Sound: 9.0
Value: 4
Comfort: 10-