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truly sweat proof
These headphones are performing as I hoped. They seem to be truly sweat proof. I've even run them under water briefly as shown in a video posted on the Sennheiser web site. The orange rubberized cabling is a nusance only because it is not slippery and catches on clothing, skin, etc. The cables are constantly tangled as a result of the rubberizing as well. The case is worthless but cute. Using it almost guarantees a tangled mess to deal with later. Also, I am not sold on the benefits of the asymetrical set up of the cable to ear phones. I would have prefered a symetrical set up or at the least, a user adjustible set up. I upgraded the ear foam pieces to the Comply brand aftermarket ones as mentioned in another review and that helped keep them in the ears while running but the sound isolation (for use on flights) isn't as good as my Shure SE-310's (at 6 times the cost!), nor the sound quality - The Sennheisers being a bit too boomy for my taste - even after adjusting the eq on ipods and iphone, but at least the Senn's still function after a vigorous and sweaty run. Something the Shure's couldn't handle no matter how careful I was with them. If you are looking for headphones for working out with and are virtually sweat proof and still have some decent sound, these are definitely the ticket. I've been searching for years for a set of headphones that will handle repeated exposure to moisture and I am happy with this product overall.

yup, they're sweatproof
i bought these in the hope they would not get sweat-clogged like the apple headphones i've used in the past. they did not disappoint. i use them mountain biking, and i can ride for hours without the music getting drowned in my sweat. they tend to shift in my ear a bit, but never really fall out -- a small price to pay, since they are far more comfortable than hard-body in-ear type phones. they also sound better than the apple headphones -- better bass. i'm no audiophile, but they sound pretty good to me. they do block out more ambient noise than i'd like, but i suppose that could be viewed as a benefit depending on how they're being used.

Fit well, impressive sound...
I've been doing too much research to find a headphone for running and finally decided on this CX380 Sport Series II. First of all, the sound quality is impressive. The bass is strong for its size. Its sound is almost as good as my HD 202 (DJ style headphone) except it doesn't hurt your ear after a long period of wearing it. Its noise isolating did surprise me because it "did isolating the surrounding noise". It fits really well in your ear and no matter how hard you run or how much you sweat, it just stays there.

The only thing I'm concerned about is the thin wire. It's very flexible and seem sturdy but I really don't know how much it would last. Gotta love its orange color though. I've had for exactly 1 month and been using it about 5 times a week. I would buy CX380 again if this one breaks!

Good for running, sound good but lack of detail in high frequencies
This is my second earphones my Sennheiser. I bought this one for bike ride and joggling because it's sweat resistance. The construction is very good, very comfortable and well design that make the earphones stays firm in my ears. Another the reason I bought this earphones was the little clip, it helps reduce the noise of the cord rubbing against your shirt. The sound quality is ok, more bass than my Sennheiser CX300 but the high freq response is weak. I guess when you are running then you don't really care much about the sound. Overall, I am happy with the purchase.
BTW, make sure you buy from the authorized dealer because I saw many fake headphone out there by looking the plug design.