Took a chance on these after reading positive comments and a review. Overall impressed, especially given the price. They take a while to settle down, initially the sound is harsh, 'tight'. I have been mostly using them plugged direct into a laptop listening to a variety of streamed music (pop, rock, jazz, classical).
After about 3 weeks use the sound has mellowed and opened out. The sound is fairly open, spacious, detailed and quite balanced / neutral, leaning towards warm, but importantly enjoyable - they seem to bring out the 'music' in songs. Overall, they generally present the music well, handling busy/raucous material fairly cleanly, with low end not usually overpowering the sound, and bringing out the nuances in recordings. Also seem to reproduce vocals, especially female vocals, nicely.
My experience of headphones is limited, the only direct comparison I can give is with Shure IEMs (SE102). The Shure's sound a little harsh and clinical in comparison, and light on bass. Also closed in, with a more limited sound stage. However, the limited bass on offer is perhaps a little better controlled.
As far as comfort is concerned, after early issues I have them adjusted so they are reasonably comfortable. They are quite large, but light. I generally use them while wearing glasses, working on the laptop, which does not help.
Regarding build quality, there is nothing to complain about for the price. Mainly plastic but feel adequately robust and are well finished insofar as there are no rough edges, bad joins or loose bits. Hardly stylish, but one can't have everything.
In summary, I am impressed with the HD668Bs. They handle a variety of types of music well, the sound is relaxed but detailed, and music is made enjoyable and involving.