I did a short comparison of the Marsall Major yesterday in HMV with my Zune 120, in a direct comparison with my ATH-ES7s.
Disclosure: I could be said to have an AT bias as I have an AD700 at home, the AD500 at work, the ES7, and the NC7. For sport I use the Sennheiser OMX70. No complaints as to sound quality with any of those, though I'm on my 3rd pair of the Senns as their durability leaves something to be desired (long story though). I also have a pair of B&O A8s which someone gave to me for a birthday present, and I do use them sometimes. They are OK but are definitely overpriced at US$120 - for US$85 (the price at which they were first sold here) they are acceptable, not least for their looks and construction.
Anyway, back to the Marshall Major. It is indeed very light and folds up very nicely. It also feels good in a "classic" sort of way, with a leather headband. I can see how it would also fit very well on the head.
However, and I may be biased, but I much prefer the sound from my ES7s, which are not exactly the best-sounding headphones around (though in my view acceptable for what they are). I only played 2 tracks, Iron Maiden's "Fear of the Dark (Live)" off their Edward the Great greatest hits album, and "The Pilgrim" off their second-last full album, "A Matter of Life and Death". Both ripped at 192K-320K MP3, VBR High using CDEx.
IMO the Marshall had more "attack" than the ES7s, in that certain things just seemed to have slightly more presence. OTOH this was at the cost of the ES7s' soundstage, which was wider and in which instruments sounded more distinct. I would go as far as to say that the Marshalls sounded muddy in comparison, which surprised me. I agree with the poster above who said that the ES7s have more distinct bass.
Having said that I do think they would be an acceptable choice for a pair of very portable headphones. I'm not going to buy them as the ES7s do a better job for me, but if I didn't have a pair of ES7s I'd be very tempted to go for the Marshalls instead, though it would mainly be for the perceived additional comfort, the interesting folding mechanism, and the "rock" factor.