Pros: Very comfortable with a mellow, involving sound that harmonises brilliantly with live recordings.
Cons: Paint chips off headband with regular use
There's plenty of debate around here as to what makes a headphone "high end". Putting price aside for a moment, I have no doubt that the HD650 marks the beginning of the high-end range. I've auditioned many headphones over the past few years and while many sound different only the electrostats such as the HE60 truly sound superior in every way to the 650s.
In terms of sound the HD650 is what I would call a "natural" headphone. Very rarely do I get the feeling that any part of the spectrum is being over-emphasised and the result is an intimate presentation with good (if not exceptional) soundstage - perfect for live performances. Some would criticise the 650s for their rolled-off treble and I believe this is valid but mainly when the headphones are poorly amplified or paired with a poor DAC unit; I would certainly avoid running these directly from a computer's sound card, for example. My pair are used with a DAC1 which does a good job of brightening the 650s' sound signature without making the treble sound sterile.
The low-end and midrange of these cans is what stands out for me. Bass is almost perfect with good extension and decent impact. Though dark-sounding I never get the impression that the bass is being emphasised to the detriment of other frequencies. I also find that bass quality improves dramatically with higher volumes, this is likely because I listen at quite low volume most of the time and the extra juice is just what these cans need to shine. Moving on to midrange, these cans are excellent for female vocals as well as string / brass instruments with the original tonality being replicated solidly.
I paid almost exactly £200GBP for my pair in 2007 and I find that to be excellent value for money. Build quality is sturdy and the sound that you get for the price is ridiculously good - others have spoken highly of the ability of these cans to scale with the rest of your system and I can only echo this. My one gripe (the reason I docked a star for design) is that the paint tends to flake off slightly around the midpoint of the headband over time; continual flexing of this point as you stretch the headphones over your head contributes to the wear.