Ultrasone HFI-650: These are very forward sounding headphones with amazing clarity and bass extension. They sound very neutral and transparent. Drums have amazing snap and punch to them, something I love to hear. The HFI-650 tends to be bright, though not overly so. Burn-in seems to have helped this, or maybe I've become more accustomed to the HFI-650's sound, but its still a bit harsh. Tight, deep bass and a forward soundstage mates these phones well with most Rock and Metal. Recommended for Electronica as well.
The HFI-650 is easily driven though benefits from an amp (what doesn't?

). The rather warm paddings, weight, and overall fit of this headphone leaves room for improvement. They can be rather uncomfortable at night, causing me to reach for the HD580. Can be a bit fatiguing. Breaks in listening every couple hours will give your ears a breather. The cord is microphonic, which is annoying while listening at night or with quiet music. Isolation is very good. You can listen to whatever you please and no one will be any the wiser. The HFI-650 seems to keep more sound in than it does out, though, and might not be sufficient for travel by train or plane.
update #1: I've just gotten my first "real" source - a Sony SCD-CE775 SACD player - and have been listening out of the headphone jack the last couple days. So far, I can say that a source makes a world of a difference. The brightness of these headphones is really tamed and the sound is just all around better and more full sounding, so that it makes the sound of my PCDP just seems gutless. More impressions to follow when I get an IC and can hook up my amp.
On a side note, I love these headphones for Rock still, although I don't think that they're really ideal for electronica anymore. The beat and energy just isn't there like it is with my Portapros. Sad but true.
update #2: About that electronica comment, I must've been feeling remarkably basshead-ish that day when I remarked that Portapros were better than HFI-650's for electronica. Portapros and even KSC-35's make my head hurt with their bass combined with electronica, and they tend to cover up a lot of the details. My ultimate electronica headphone still awaits, but for the record, I do like the HFI-650 for electronica.
Jesus I feel like LTUCCI...er....

I also still need to get that mini-RCA adapter to use my amp with them and my SCD-CE775. In a sense then, yes, my sig is lying, but it makes me both look and feel better