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Possibly the most important question for most potential buyers is 'how do these compare with the stock iPod headphones?' The answer is 'rather well, actually'.
Don't expect better aesthetics, because you won't get them. The smooth curves of the iPod phones aren't replicated. Instead you get functional straight lines and creases. Then there's the gray paint finish, which is a tad dull and prone to scratching if the simple (but surprisingly useful) case isn't used to store them. Build quality is sound though. Cables are well soldered and can withstand a fair dose of careless treatment (e.g. getting accidentally yanked after being wrapped around gym equipment, as I've done several times).
The best bit is the sound. To me the standard iPod phones sound a mite laid back, not particularly expansive and they tend to muffle too much detail. These Sennheisers are more efficient ( so they sound louder and clearer at a given volume level), they open up the soundstage significantly and provide a healthy extra serving of definition across bass, midrange and treble frequencies (although I had to put the cloth covers on to get a good in ear fit - which upped the bass response no end). They're not as rich or precise as a pair of Sennheiser PX100s, but those are normally double the price and they're less good at overpowering background noise.
I'd recommend them strongly - with the caveat that on a Creative Muvo MP3 player playing music encoded in WMA they sounded too bright and harsh for my taste - but I reckon that was more a consequence of using WMA than the Creative player. Stick to well encoded MP3s on an iPod though and you'll be impressed