I'd suggest at least trying them with a LOD cable (if you use portable devices) and portable amp. The Headroom Total Airhead can drive them with ease. That's only $49 if it's still on sale.
Right now i'm loving the HD-650 with the Airhead from a Ipod Touch 2G with the Fiio L11 LOD connector. This setup is finally complete after trying every portable player I could find..
I know this will hurt the Ipod Video 5.5 fanboy's feelings, but I think the Ipod Touch 2G sounds better with the HD-650. The 5.5 (and Clip+) are too warm for the HD-650. At least for my preferences.
I wonder what makes the Ipod 5.5 sound too warm? I guess it doesn't measure flat. I'd like to try the Ipod Classic 7G sometime. I had an earlier 160gb version previously. Got the Ipod Touch 2G as a gift when the 3G came out.
BTW I've found the HD-650 to sound good, but not great from portable devices with no (external) amp.
Hmm... well I don't use it with any portable device. Mostly with my laptop which has a realtek alc275 card (not sure how good it is).
And to those saying, just hold on to it... If i sell it i'd likely buy something else like a mirror-less camera with the money.