I've used these for trail and road running approximately 100 miles now.
Overall: Great bass, fair to poor highs, great fit, but has a few functional issues.
Even though the highs are diminished, the bass and overall sound is still better than any headphones of this type I've ever heard. The fit is excellent. It is lightly spring loaded to hold each earbud in your ears. It never slips and is never uncomfortable for me for over 90 minutes at a time. Car traffic can still be heard pretty well.
The mic and controls are well-placed, but hard to distinguish the buttons while running. I've taken maybe 4 phone calls while running, and everyone has complained about wind noise and basically unusable sound quality. The cable is on the left side, which makes it a poor choice for a hands-free set while driving for me. The behind-the-head frame probably makes these a poor choice for use on an airplane, but I haven't tested that. I have rinsed them with water several times with no change in performance.
Two biggest complaints: Excessive wind noise while running or in any light breeze, and foam pads that constantly fall off. The wind noise is usually tolerable, but often not. The foam ear pads are a must if you sweat at all. Without them, the rubber earbuds become like toilet plungers on your eardrums. Not good. The foam pads mitigate that problem, but they are far too delicate and easily fall off when taking the headphones off. I have dropped the little foam pads on the trail several times, but miraculously found them again. Sennheiser needs to fix that and they need to give you at least a half dozen spare pairs.
I do like these, but for the price, they should not have the problems I noted.
i always go for small ear-buds-on-a-wire... fit my irregular head well and stay put when i am running. i've been through many generations of Sony clip-ear or back-head ear-buds. they were good, but disposable. i jumped up to the Sennheiser's on a money-lark...
these things are expensive, but well-worth it. incredible bass and dynamic range. i'll never go back to lesser ear-plugs.