Pros: spacious, good separation, natural and even across the spectrum
i've read a number of reviews around the net which criticize the k812's for harsh treble and limited bass - all i can say is that my experience with these 'phones does NOT bear that out. the bass criticisms appears to come from people who want to listen to dubstep. i don't know what dubstep is. the treble criticisms come from people who are listening to some types of pop music with which, again, i am not familiar.
i am using these phones sourced out of an ibasso dx90 i am listening ONLY to classical and jazz flac files, and the sound is wonderful.
for example, i spent time last night comparing 4 versions of schubert's "death and the maiden" - and the source and phones revealed significant differences among the performances. they revealed not only the obvious ones of tempo, phrasing, and expressiveness, but also differences in how the groups were miked and/or sound mixed/engineered.
the lindsays sounded as if they were recorded while playing in a warehouse, with an open, hollow and slightly distant tone, especially for the cello which sounded a little muffled or veiled. the alban berg quartet and the takacs had a better sound, very clean and present, while the melos recording made me feel like i was sitting amongst the players, and could feel the bows being drawn across the strings. the high notes in the andante movement were intense, expressive and clear- very much revealed by both the source and the headphones, without any sibilance or "tizziness." in the melos version, the cello was rich and more present throughout in a way that i very much enjoyed.
i then spent an hour or two listening to jazz. charlie haden and pat methany's version of "first song (for ruth)" showed off haden's bass work. the bass notes were deep and rich, the sound of the strings being plucked very clear, while also being tightly controlled without any loose or blubbery sound. the piano on jacky terrason's "nardis" was very much present, not at all recessed or veiled, across the full spectrum of the instrument. some brazilian jazz-pop by maria rita carried the vocals forward. they were lively and caught her occasional slightly gutteral or raspy sounds.
through all the music, classical or jazz, the phones presented a spacious soundstage, with good separation of instruments, but with the size of the soundstage varying with the recording. in the schubert, for example, the lindsays had a 180-210o wide soundstage, while the melos quartet was spread over only 120-150o.
bottom line: i'm very happy with these headphones. i can't speak to their application for other forms of music, but for what i want to listen to, they're great.
edit- to add a note about comfort- i listened for several hours yesterday and there was no fatigue. they're extremely light and comfortable, so much so that i didn't even think to comment on it.