Oooh the fancy earphones

What you should take away from this review.
The ER-6i is in a different class from most cheap headphones. If your headphones normally cost U$20 or less, these have much better sound. Once you try the ER-6i, you will not be able to enjoy cheap headphones.

Ease of Use
The ER-6i comes with an instruction booklet. This booklet must be read to use the ER-6i properly. This means the ER-6i is so difficult a normal adult has to read a book to figure out how to use it. Unfortunately, your reviewer is not that smart, so he had to check the on-line video that explains how to insert the ER-6i. Even after the book and video, I could not get a reliable seal with the ER-6i preferred rubber tips. The ER-6i uses noise isolation, and the tips should form a seal that blocks out most external sounds. Excluding these sounds allows you to listen at a lower volume, since you would be listening in silence. The booklet states that the a proper seal is needed for good base response.

I could not get a good seal with the rubber tips, even after clearly following the instructions. The shape of the tips did lead to some discomfort when inserting them. And were quite filthy when I removed them from my ears. The foam tips are familiar "technology", similar to most ear plugs. They are good for about a month, and okay for two months. I wear my earphones 10+ hours a day everyday, so you may use them for a longer period. Replacement foam tips cost U$15 for 3 pairs at the Etymotic website, so they are not cheap. The foam tips like the rubber tips are inserted in the ears, so their is a bit of discomfort.

The ER-6i includes a tool kit for replacing the filter, it includes an inch long metal pin and other tiny pieces. I am "looking forward" to the day I need to replace this filter.  The other parts of the ER-6i are mundane, there is a simple alligator like clip which works fine.

Sound Quality
I don't like the the rubber tips, I don't get the proper seal and I find them useless. The remaining comments deal with using the foam tips. The ER-6i blocks external noises, when the foam tips are new you can stand on the road side and not be disturbed by traffic. This means that you should be extremely careful with these in traffic. You should not jog or ride in traffic with any headphones, wearing these would be suicidal.

On the positive, blocking the sound allows you to turn down the volume. How much? You can turn it almost off, and still hear the sound clearly. This is why you can't switch to a cheaper headphone, because you are aware of how the increase amplification is destroying your hearing.

I am not an audiophile, and I listen to more spoken word than music. With this caveat, I love the sound of the ER-6i, I think it is clear as a bell and the music sounds great. The only problem, is the lack of bass. The booklet insists a good seal will improve bass, but I find this facet sadly lacking. There is also a problem with the cable because any movement can cause an irritating noise. I observed this problem with my W810i's headset, so I will assume that this is a problem with noise isolating headsets.

Build Quality
The cable looks like those found on cheaper headphones, they seem likely to break. Especially troubling are the connections to the speakers which seem fragile. I hope they last, but I doubt they will be around for two-three years.

I paid about U$65 for my ER-6is, considerable less than the MSRP of U$140. I think they are worth about U$100, since they should preserve your hearing. I would recommend, with two caveats, bass sucks and the headphones must be babied.