This product and the similar Sennheiser CX 880i Premium Ear-Canal Headphones with Microphone & Smart Remote earbuds have great sound. The 880 has the integrated volume control, and the 880i has the iPhone/iPod compatible mic/volume attachment. (Even though that product is designed for iPhone/iPod, the mic and pause button -- but not the volume buttons -- work with my Blackberry Bold 9700.) There are numerous thoughtful details in this product, and a couple of significant shortcomings which I'll describe. Overall, this is a very excellent 4-star product.
The sound is just great. You can get earbuds with better sound, but it won't be cheap. I listen to all kinds of music including many genres of popular music as well as classical, and the sound with all is very smooth. The highs are very open and clean. Some listeners may prefer to slightly boost the midrange. The bass, including the deep bass, is solid without using any bass boost. I hear a small peak in the mid-bass. I make very *slight* adjustments in my iTunes equalizer to make the already great sound a bit better.
The fit will depend on how well the included selection of tips fit your ears. In my case, I found tips that fit very well. They stay put very snugly, and they're very comfortable even for longer listening sessions. The shape of the ear pieces is angled, and that works out very well. The way the cord exits the ear pieces is also pretty good.
These are designed to be good for noise isolation, and Sennheiser pulls this off really well. Note that noise isolation is different from noise cancellation, but the end result is very similar. With both, you don't hear outside noises and you can play the sound at lower volumes (which helps to protect the ears). In my experience, noise isolation works better at higher frequencies, and noise cancellation works better at low frequencies (like airplane noise). I've already used these earbuds on one flight, and I'm happy enough that they'll find a home in my travel kit.
Speaking of the travel kit, the included case is surprisingly well designed. I love it. And the little cleaning tool really does help to keep the ear pieces clean, along with the well-designed diaphragm covers.
As other reviewers have mentioned, there are significant microphonics issues with the cord. This is disappointing, since the sound is otherwise so excellent. I've found I can eliminate most of this problem by stuffing the middle of the cable inside my shirt a bit, so there's less loose cable to rub on my collar. That works pretty well. Still, a different material for the cord could solve this better.
Also, the cable feels a bit fragile, especially where it joins the earbuds themselves. I've been using this less than a month, so I don't know how this will fare in the long term.
On balance, these are easily the best earbuds I've ever had. I'm finding new joy in listening to music when I'm out and about, or late at night when I don't want to disturb the rest of the family.
*** UPDATE ***
Six months later, and I'm still very happy with these earbuds. There's one problem I've grown very tired of -- If you don't put them away properly in their case, the cord gets badly tangled. It takes much longer than it should to untangle these guys. Also, the plug end has become noisy where it attaches to my MP3 player. Same problem on 2 players, so it's the cord for sure. Not too bad yet that I want to return it; not sure if it will get worse over time.