I use the Shure E3c's with my new 4G iPod (see signature for details). Overall, they're great. It took me a few days to figure out how to insert them correctly to get a good seal. Now that I know how to get a tight seal every time, they block almost all outside noise. They are light on bass, but for almost everything else they are outstanding. I notice parts of music with these earphones on that I never have before.
I haven't heard the Etymotic ER-6's (nor the ER-6i), but they've gotten great reviews. The ER-6 is rated slightly higher than the Shure E3c on CNET - 8.3 vs. 8.1, I believe. The Etymotic ER-6 retails for $130 and the Shure E3c retails for $180.