Cardas, because it does the job so well that I don't see any reason to upgrade further or try alternate cables; its what I have and I'm happy with it.
PROS:
1. Considered to extend the detail of the HD580/600/650 to much higher levels
2. Excellent imaging
3. Very airy, precise, expansive high end
4. Tightens and deepens bass quite a bit
5. Has absolutely no problem fitting into the HD650, even though its designed for the HD600.
6. Not very microphonic.
7. Light in weight and reasonably flexible.
CONS:
1. One hundred fifty bucks is a lot to spend on a cable; then again, it's an improvement that might do as much for you as upgrading your amp.
2. Some users claim other cables have better bass performance (Zu), a smoother richer sound (Equinox), an overall more balanced, detailed sound with a fuller midrange (Silver Dragon), or are cheaper and offer similar performance (Meier).
3. Cable upgrade will not sonically change the HD650's balance. The HD650 is going to just open up a lot but wont become a different headphone.
4. Reveals crap further down in the audio chain; makes my PC audio sound terrible but makes a vinyl setup sound as vivid as being in the hall itself.
5. Need to buy a 1/4" to 1/8" converter if you are plugging into a 1/8" port of some kind.
6. Lengthy break in time and tends to change, sonically, if lots of movement occurs to the cable.
Cheers,
Geek