Since I travel on the train to work every week day I thought I should invest in some good quality ear-phones for my iPod. The standard ear-phones are acceptable, but when I felt the need to turn up the volume I could tell they weren't healthy for my ears.
I chose Sennheiser for being a reputable brand, and the IE 6 has an audio range from 10 Hz to 18,000 Hz. At around AU$200 they were a bit of an investment, but I was relieved to find they were worth it.
The IE 6 is noticeably superior to the standard iPod ear-phones. The sound is clearer, and much bass-ier. Previously I would tweak the iPod's sound settings because the speakers would distort the sound. But with the IE 6's the speakers don't cause any distortion, but you still tweak sound settings to get the best sound.
Beyond just sound better, the speakers are overall louder. I normalise all my music with MP3Gain which effectively reduces the volume of all my music. With the iPod ear-phones a comfortable loud volume was pretty close to the maximum iPod volume. With the IE 6's I get that same loudness at lower iPod volumes and if I choose to go louder it doesn't feel like I'm damaging my ears with the IE 6's.
I had never used "in-ear" phones so I was concerned I would find them uncomfortable, but this wasn't a problem either. Since they are "in-ear" they isolate a lot of outside noise--great for use on public transport, not so great for hearing approaching cars when crossing the street.
The only con with these ear-phones is that I have so much trouble spelling "Sennheiser".