Comfortable as in the noise level doesn't hurt your ears. The beauty of the Etymotics is that they fit so far down into your ear canal and isolate so incredibly well, you don't have to turn the music up very loud at all and you get the effect of much louder music than you really are getting.
For instance, I just got my ER-4S yesterday. Before that, I was using the RE-262 with Comply tips. When I was done listening to music for any length of time with the 262's...when I took them out of my ears, it took a while for my hearing to return to normal because I had to turn the volume up to get the sound level I wanted. I was worried about the Etymotics being worse since they fit so close to the inner ear, but even after 5 hours of listening last night, when I took them out, my hearing was normal. I attribute that to the much lower volume needed to get the sound I wanted. Using a cMoyBB amp, the Ety was 1/4 of a turn up from 0 opposed to 1/2 a turn up from 0 with the 262...so there's a pretty big difference.