I just picked these up. I am no audiophile, and don't know the lingo, and I apologize ahead of time if my choice of words is muddy.
Listening to these over a balanced cable on a Sony Walkman NW-ZX300. The treble was a bit much for me, so I EQ'd 2 kH by 4 dB and 1 kHz by 2 dB, as is commonly reported in reviews. Made a world of difference.
The separation of instruments is extremely defined. On some tracks, it really enhances the track. On other tracks, compared to the iSine 20, the definition of instruments on the FH5 is almost fatiguing. The best way to describe it is hearing a concert as each instrument rather than as a combined single orchestra. Like I said, though, with some tracks, it's really good and preferred.
Bass is tight, not boomy, but pronounced. I'd say compared to the iSine 20s, it's not quite the same "quality" of bass.
Mids are good. Vocals can be really in your face (forward) with some female voices. The EQ mentioned above helps. Instruments - esp woodwinds - sound very well defined. Adele sounds good.
Treble can be a bit too bright on some tracks for my tastes, and I like metal and prog rock. Listening to RUSH right now, and there are points where the highs are a bit much - again, compared to my iSine 20 and also my AKG K7xx. Some Iron Maiden tracks have the same issue. Interestingly, vocal/choral is spot on. I can listen to these all day. Just spun up some Hidlegard von Bingen, and sounds really good, and that definition ensures the voices are not muddied in the very sparse instrumentation as I've heard on some other IEMs.
Build quality seems to be really good. I haven't used the provided cable, so can't comment, but the actual units feel solid. Again, no audiophile, so I don't have a lot of experience. I will have to try the small tips, though, as the mediums feel to be right on the verge of too large and they make my ears ache after a short while.