Here's my unscientific (shoot from the hip) report card on headphones:
Sony 7506--suckout in 'warmth & body' region (200-400 Hz); bright & clear but 'skinny'
Fostex T20--darkish... they'll take some punishment. They seem amplifier-dependant somehow: they sometimes sound good, but sometimes lackluster
AKG 240--generally good tonal balance, won't take a lot of punishment; there have been stories that the drivers are wired out of absolute polarity, meaning that singers--because they also hear themselves physically--will hear their reproduced voice as thin when both sources combine. Have not confirmed this, but it is a concern.
Sennheiser 280 Pro--initially liked these a lot. Great isolation, pretty good tonal balance and probably, in the long term, I will prefer these to any of the headphones listed above. Bottom not terribly extended, but overall more natural frequency response than he 7506's and Fostex's. Probably will prefer them to the AKGs as well, but the jury's still out. Not good enough/flat enough to mix on, but they are fun to listen to.
Grado 225--used these for years. Open back so no good for tracking/overdubbing (generally speaking). The original flat foam earcups are more accurate in the low end than the recessed ones. Good-sounding & pretty extended in both directions. Although they served me well at the time, they have been retired for several years, originally replaced by:
Sony MDR-CD3000--pretty great headphones. Leaning just slightly towards the bright side, they also accentuate sibilance even a tiny bit more than their slight brightness. Other than that, good, apparantly flat response and very extended on the bottom end, excellent isolation from leakage. They'll play fairly loud without strain and are extremely comfortable, although a bit big & bulky. Unfortunately I think they have been discontinued.
Sennheiser HD600--open backed, these headphones have, by a good margin, the closest tonal balance to my 4 way custom monitoring system (and treated room) on which I have worked incredibly hard. If you like, and understand, Tannoy Golds with Mastering Lab crossovers, I predict you could comfortably and easily mix a good-sounding record using only these headphones. Bottom end is quite extended and fairly powerful. These are my new reference. Not as comfy as the Sony CD3000s, but close. I'm pretty sure George Massenburg said he likes these (the Sennheisers) a lot when he was still hosting a forum here. I love mine.