Pros: Quite balanced sound, lightweight, build quality
Warm, good bass extension, quite sweet highs, quite balanced sound, probably with slightly emphasized mids, not airy, recessed 300-500 Hz and 7-8 kHz. I think that the sound signature is quite similar to Audio-Technica ATH-ES10's sound signature but it's more mid-centric and not airy. The clarity is very similar too. I like the mids that are really natural. So if I can say that the ATH-ES10 is the ideal portable headphone then the ATH-ES88 is almost ideal because I think that the difference is not big.
Comfort and build
The build quality is amazing, it is difficult to scratch the textured aluminium housing, this unusual pivoting mechanism is very reliable, flat cables. I like the design very much. I can't say that the ATH-ES88 is comfortable: driver shells are placed not deep so it can hurt your ears. I can use it only three or four hours max.
I do agree that ATH-ES88s are very underrated.