I bought these to enjoy and to assist development of a headphone amp, having owned Sennheiser phones in the past, the last being HD518s. First impressions on unpacking are of fairly ordinary appearance and materials although actual comfort is first rate. First impressions of sound are that they sound a little thick and subdued. I can understand why people could be disappointed; there are plenty of cheaper phones which impress more for the first minute or two.
 
However these are dark horse headphones. Although their overall presentation is mellow, and some would say veiled, they are incredibly revealing in reality after prolonged listening. Get the source and amplification right and they even do rock really well. They are not really suited to mediocre source equipment since they add little drama or coloration to things. The midband is remarkably even and subjectively lacking in obvious resonances. You will readily hear the affect of altering items in your source chain, and indeed in changing parameters and topologies in your amplifier. Although the balance is warm, they are not that forgiving of poor source material. Instrumental timbre is rich and detailed, although, again, it tends to take time to appreciate this, as it is not presented in a way that grabs your attention immediately.
 
Voices are cohesive and have natural body. Here IMO it is easy to spend more and get less. Power handling and linearity at high levels are excellent, although at 300 Ohms you will need volts to get them going; portable devices are unlikely to satisfy here. On rock, they actually tend to come alive at higher levels; there are probably better choices if you prefer to listen quietly. Leakage is predictably high. 
 
A 'mature' design but definitely still one with a lot of appeal.