Having read many good review of these earphones, I thought I'd treat myself and go for a decent-ish pair of earphones I could wear running or whatever. I normally listen using a pair of grado sr60s, and of course didn't expect these to match up at all.
However these earphones are way way below what I expected. The bass drowns out all higher responses, they sound horribly 'muddy'.
In addition these are the worst fitting ear-canal style phones I've ever owned. I used to use a pair of creative ep-630 and despite being very similar, they are by far and away more comfortable.
The last negative about these phones is that there is a stupid in-line volume control (variable resistor) that you can hear is clearly affecting the sound quality when you slide it (the sound crackles A LOT). It's got to the point that the resistor has developed poor contacts and now one earphone is louder than the other. AAAAH!!!!
I like these earphones because they have a symmetrical cord, and a 90 degree headphone jack.
I've now switched to a pair of creative ep-830 earphones. These have a noticably better sound than the ep-630 model and by far and away a better sound than these cx500. No in-line volume control, comfortable, cheaper. Top stuff creative! Sennheiser, sort it out!
p.s. if you're about to buy a pair of cx300s, then go for the ep-630. They are cheaper, and the only real difference is that they have a symmetrical cord, unlike the cx300s. Hope this helps!