Two things can immediately improve the sounds coming from your ipod:
Indeed the better the headphones that you have, the more you'll hear the artefacts of lower bit rate encodings. So, suppose you have some nicely encoded, or indeed entirely lossless tracks on your ipod (try putting your fave album or set of tracks on your ipod imported direct from the CD rather than encoded. Play them up against encoded mp3's or aac's and see what you hear) and you'd like to hear as much of what's there as possible: what headphone style do you choose?
You can choose traditional cans (regular over the ear headphones) - Aside: Next time you're thinking about sony or seinheisser, give Ultrasone HIFI or DJ phones a try - hit "test reports" to read reviews - they're designed to preserve your hearing, too). These types of headphones are dandy for certain kinds of situations - long listening sessions; editing - but for being on the go, they're bulky to say the least, and not great if you want to fall asleep with your tunes.
Another style is the popular earbud phone that comes with most digital audio players like ipods. These sit levered in the ear. In ear phones, like apple's in ear ipod headphones, point more directly into the ear canal, and thus cut out more of the noise from the outside, improving perceptions of things like bass in particular.
Etymotic ER-6i
The next level up (or in) is full into-the-ear-canal phones. When you see musicians on stage at live events and see the lines leading up around their ears, you're seeing custom made in-ear monitors (iem's) where the earpieces fit into the ear canals . The custom fit isolates external sounds and directs sound from the monitors into the ears.
And this is how good headphones can save your hearing: by having the 'phones cut off outside noise by over 20db, you can turn the volume DOWN on the listening device. Add to the isolation effect great transducers to translate the impulses coming from the source (like an ipod) into great sound, and you have a super combination.
The etymotic 6i's, as reviewed by the formerly named "ipod lounge" represent an affordable combination (approx 140 USD/75GBP) of great (hearing-saving) isolation and super sound, designed specifically with digital audio players like ipods in mind. What does that mean?
Etymotics makes a number of great in-ear sound-isolating phones: the 4 as flagship, the 6 as more affordable but still cool cousin and the 6i for ipods. The main thing about the 6i over the 6 is that it provides some much appreciated bass boost, as shown in the chart comparing the 6 and the 6i. This boost is subtle - you won't experience it as the bass pumped up, but as an overall richer sound, where you can better pick out the distinct bass and drums textures from a track.
What sets these headphones apart from other "in ear" phones, however, is the kind of audio detail available across the spectrum, not just the bass. You may hear instruments and touches that you hadn't heard before. In vocals, i've found that i'm hearing much more of the expression of a phrase or a note than with standard earbud or apple's in ear phones or sony's in ear phones (any model) or many traditional cans.
Moving in the world with 6i's on: Isolation of in-ear phones like these means that you do not hear things outside yourself - if you're running or just walking down the street, you may want to think about not putting these in so they're fully seated - that will let you use these more like ear buds where you can still hear the traffic.
Getting used to Canal Phones. Some people do not like the idea of sticking something deep into their ears. If you're one of these folks, these headphones mayn't be for you. One way to check your tolerance is to try out a set of disposable ear plugs: how do you feel about putting these into your ears? If you're ok with that, you can be ok with the ety 6i's
The 6i's come with one set of flanged ear pieces and one set of "memory foam" ear pieces (same stuff used for ear plugs). Each fit slightly differently. Other sizes of foam or flanges are available and etymotic will send you a sample pair different size if you think you need it. You can then get sets of the sizes you want. The degree of isolation is also different: 22db with the flanges; up to 30db with the memory foam. I've found i like the feel of the flange better than the foam, and for the environments i work in, including planes, the 22db is more than adequate.
Fit is critical to the sound experience of these headphones. Some people have said they've made their own modification to the 6i's flanged ear pieces cutting off the top flange so the larger two are left (scroll down this review on ilounge to see comments by people of their pre and post mod experience of the phones - all post mods are "wow these ARE awesome." - perception of the bass is particularly noted.)
Isolation vs Noise Canceling/Noise Reduction. Another attribute of the 6i's or indeed any good isolating phone is that it is better at reducing plane/train noise than "noise cancelation" headphones like the popular bose model - which also require batteries.
Now, a lot of people are happy with their £275 Quiet Comfort bose phones even though their web site does not say how many db's of sound are cancelled, whereas Sony's 99USD fontopia earbud noise cancelling headphones make clear they offer only 10dbs (70% of noise); etymotic's foam ear plugs are 30dbs; their 6i flanges are 22db.
For £80, with etymotic ER-6i's, you'll get both a better sound floor and better sound isolation in a lighter package than either Sony or Bose and one that doesn't require batteries. If you don't believe that, try this simple test: go somewhere where you can try a pair of the bose quiet comforts. Try them on, turned on. Then, remove headphones and stick in a pair of memory foam disposable type ear plugs (you don't squish in these: you roll them first to kinda compress them, then insert into the ear and hold for 5 secs: they expand to fit the ear canal shape). Which produces better noise reduction? If it's the ear plugs, imagine great sound coming through those plugs...
Aside: If you want to spend £275 on in-ear/isolation headphones, though, you can, and you can do much better in terms of both sound and isolation with that cash. See the Next Level section further down for how-to's. For about half that price, you can get some awesome professional level audio headsets, too!
Any Negatives? Hate the case. Ok, hate is a strong negative, but really. 
Etymotics provides a wee neoprene pouch for the phones with a mesh divider. This means stuffing the cables and earpieces into the thing. It's a mess and timely. ( See update bottom: the case has been changed and is a much better zip case)
An alternative device for storing 6i's is the earPod - a generic earphone case. Not quite as cools as sony's winding cases that come with their own headphones, but more durable, and will definitely fit the etymotic 6is as shown in this review at ipoditude.
Overall. The 6i's are a very good set of headphones. Whether you're listening to classical or house, the degree of detail in sound is impressive. The better the encoding the better the sound, of course, but even with lower bitrate encodings, these phones improve the experience.
The isolation is a boon for being able to turn the sound down and still hear an impressive and enjoyable level of detail. The isolation is also great for cutting out travel noise of the train/plane.
They take a bit to get used to in terms of learning how to put them in and get them out; and as other reviewers have noted, the difference between great and crap sound experience is all in the fit. Taking the time to get that fit right, though, pays off.
If you are looking for
- an improvement in audio experience on your ipod
- an improvement in your travel-with-audio commute experience
- a way to ensure the ongoing protection of your hearing
then these phones are a great option.
The Next Level: "Pro" IEMs
At the start of this article, i claimed these 6i's are an "affordable" package worth the price. Well, at around 150USD, "affordable" may be eye popping for folks used to thinking in terms of 20 bucks for earbuds. But that price is only the beginning.
