The HD650 sounds awesome. It's a more hard-hitting, smoother, more neutral sounding headphone than the HD600, preserving its greatest qualities - balanced, reasonable sonic presentation and incredible detail.
I spent two hours listening to the HD650s with a blockhead and meridian 588 today. These headphones definitely have some potential, and when driven by a monster amp you get speaker-like imaging and one heck of a good soundstage.
The HD650s are real keepers. They sound better even on relatively poor equipment, but as always, if they disappoint, make sure you aren't outclassing your CDP and amp with the senn phones!
Cheers,
Geek