I have both the ER-6 and the ER-4S. I started with the ER-6 and then recently moved to the ER-4S. They are great headphones. I also have Sennheiser HD600s and the ER4S's rival them. I use the etymotics when traveling and they provide a significant amount of noise reduction. I've tried some of these "canceling" headphones and they just don't offer near the amount of noise reduction you get from the etymotics. The only negative comments are that with a sealed design, vibrations from the cord can be transmitted to your ear. So if you wear these while walking, and the cord brushes against your shirt as you walk, you may hear noise. Also, I have found that the cheaper ER-6's fit in my ear better than the ER-4s, which have a different design.
Also, do yourself a favor and consider buying an inexpensive headphone amplifier. You can buy a portable one for $150. This will make a much more significant difference in sound than the headphones. Also, if you are going to spend the money for ER-4's, don't buy the low impendance version. Instead buy the 4S and a portable amp. You won't be sorry.
Here's a site that has info and some good prices (no relationship).
http://www.headphone.com/layout.php