Pros: S-Logic, ULE, sound quality, punchy bass, treble
Cons: uncomfortable, huge headband
I really love these 'phones due to their bright sound, extended treble and punchy bass.
The frequencies range from a very deep sounding sub-bass to almost harsh highs, which to me sound very sparkling, though.
Whenever I wear these headphones I never have the feeling that the sound is altered and they feel very natural to me. With warmer sounding cans like the Beyerdynamic, I have the feeling that I cut myself from real life and I can easily tell if somebody is talking to me. With the HFI-780, outside noise could just as well be part of the record - I couldn't tell the difference with people speaking in the same room (with live recordings anyway).
I ripped my eardrum open last year, that's why I decided in favor for Ultrasone's products and their S-Logic (Plus) models that do not send the waves directly into your ear but actually onto your ear, which creates a more distanced sound. My doctor said, however, that earphones can never be as loud as to damage the eardrum so in my case, at least, S-Logic is but a sound preference.
ULE metal shielding sounds fancy and it's slightly calming to know it's there, but in the end it might be just another gimmick.
However, the S-Logic sound makes the music sound much further away. In case of the HFI-780, soundstage is great and separation of instruments pleases. S-Logic seems to be a "love it or hate it" kind of thing and in my case, as I love it, I have a very hard time to adapt to IEMs for portable use.
The sound signature is mostly recommended for electronica and other electronic genres of music. However, I'm very pleased with the sound when listening to Neo-Classical (violins especially because of the sparkly treble), Vocal (great resolution) and Rock (punchy bass is magical for kick drums). Ironically, I mostly listen to Jazz. Although soundstage and timber get the most out of great recordings, the overall sound signature could be warmer for Jazz and the bass boomier for sub-genres like Lo-Fi, Ambient, Jazz-Fusion, etc. But as soon as a high note kicks in I forget about that!
I've often heard that people think the mids are recessed and bass is too strong. I can't agree at all and also looking at the frequency graph I can't see why people think so. The DT770, for example, has way(!) more emphasis on bass than the HFI-780.
Speeking of Beyerdynamic, comfort is a field in which Ultrasone as a lot to improve in. The majority thinks that the pads are too stiff and thus the pressure is uncomfortable. I have a small head and I can't agree. However, the headband is too big for me, so that I've put a piece of cloth between the headband and my head to prevent the pads pushing my left ear down. Others say that the headband does not extend far enough. Everybody says that it's not comfortable enough.
Like I've stated before, this is a love it or hate it headphone. Personally, I've fallen in love on day one and I don't see me switching to another brand anytime soon. I'll eventually upgrade to another model because of comfort issues, but soundwise I'm fully pleased. Personally, I think this is the way music is meant to be heard. I really can't speak in favor of darker sounding headphones because the HFI-780 nailed the sound of a violin, which is my favorite Classical instrument.
Edit/Update (03/2011): I've had the HFI-780 for almost two years now. Naturally, with increased experience and testing of different models, my perception has changed or evolved.
As for what I thought was a great timbre, I still do think it's appropriate for the price level, but it's definitely not it's strong point. Once you get used to the HFI-780, instrument separation is easy and instruments sound natural, but if you switch headphones often, the timbre might be lacking a little authenticity.
Meanwhile, I am a bit bothered by the recessed and flat mids. It's not a biggie, but something that will grow apparent after some time.
As for the comfort, I have replaced the headband's pleather with an Ultrasone velours strip, which improves it a lot.
Finally, these are still a great set of cans, especially for movies and I couldn't find a better headphone for the same price.