Pros: Great sound, unbelievable isolation
Cons: Poor audio output, lame bass
These were my first in-ears, and since owning them I've had the Ultimate Ears Triple Fi and the Klipsch Image S3s. I'll be comparing the ER6s to these.
I remember the Etymotic's sound being very warm and natural, absolutely never harsh sounding. You could throw it the hissiest, most cymbal-laden music and it would sound wonderfully dark. It really had a unique sound that took me a day or two to get used to, but was worth it. In comparison, the Ultimate Ears were much a little colder, and the Klipschs are tinnier.
Now despite my love of the Etymotic sound, there are some problems. The bass isn't present enough, for one. I know, I know, that's what everyone has to say about the ER6s, but before you write it off, be aware that the bass is adequate and the sound is not thin, it just doesn't have low-end punch at all. For rock, even metal, this isn't so bad. Where this is the most apparent is on electronic music, where you sometimes want really loud, punchy bass. It's not a dealbreaker, but it is a flaw. Another thing to realize is that I had the ER6's, not the updated ER6i's which are supposed to have better bass output.
The other big annoyance was sound output; these things just BARELY get loud enough, and sometimes don't. I found myself listening to these at 90-100% on my iPod, despite their excellent isolation. And that brings me to isolation. Here, the ER6s really, really shine. They absolutely blow the Triple Fis out of the water in terms of isolation. If you want noise isolating headphones, baby these will work just fine.
I ended up giving these away when I found a great deal on the Triple Fis, and while I like them both, I think the ER6 is much better value. If you can get past the low output level and the anaemic bass, these are really special earphones.