The worst!
Sennheiser MX55VC Twist-To-Fit In-Ear Stereo Headphones w/ Volume Control

Expensive --- VERY expensive for such a shoddy product.

Sound quality --- Dreadful! Either all bass or all treble --- no mid-range at all.

Cords --- all twisted up like a corkscrew --- very annoying. Earpiece cords are uneven lengths --- even more annoying, especially since it sets the in-line volume control off to one side, making access more difficult.

Ear buds --- The so-called "twist-to-fit" is a joke. I played around with them for a half-hour and couldn't get a better fit, no matter how much I tried.

Carrying case --- Pathetic! You have to put this together yourself and it is ugly beyond belief.

In short, the worst headphones I've ever owned. I will NEVER buy Sennheimer products again and will actively campaign against them to friends and acquaintances who might be considering a new pair of buds.

Amazon only allows posting with a One Star minimum, but I actually rate these ZERO STARS!

Sennheiser in-ear sport headphones
I love these sport headphones; once they are in, they don't move; very secure and the sound clarity is very good; the color is great too!! Great features and easy to fit. I'm very happy with this product.

Great for workouts
I purchased these headphones specifically to use while working out. They did take a little getting used to at first, to get the right fit, but now they only take a few seconds to put in. I'm very happy with the sound quality and the way they stay in my ears even during rigorous activity. I really like the fact that they don't fall out of even move when I'm wiping my face with a towel. They do tend to hurt my ears after wearing them for over an hour, so I wouldn't recommend wearing them for very long periods of time. I do think these headphones will fit everyone differently, as people's ear shapes can vary. I have all of the smallest attachments on and they are still a little bit too big for me.

To address a few complaints that I have seen in other reviews:
1) The lengthy and 'twisty' cord - I use them with my iPod shuffle and find it very easy to leave the cord wrapped up (similar to the picture) and clip it into the iPod's clip.

2) The different lengths of the left and right headphone cord - This is by design. The right cord should be wrapped around over the left shoulder and behind the neck. This is so you only have one cord hanging in front of you.

I've been using these headphones everyday for a month now and overall I am very happy with them.

good sound bad cord
I bought this to replace one of their earphone sets I really liked. This has good sound, but the cord is to long and it is coiled. Every time I take it out to put it on I have to spend time to unravel all the knots. The previous style of earphones worked really well, but they don't sell it anymore. They should go back to the styles that work.

pretty much perfect
this is the 6th set of earbuds i've purchased for the ipod.

finally, buds that stay put in my ears! this is the design that ALL earbuds should adopt. put them in the ear and rotate the extra little nub so it wedges into the ear shell, and you won't have to repeatedly jam the buds back into your ears.

i thought i was the only one having trouble with traditional buds constantly falling out of my ears, eroding the ipod experience. i've since discovered that most everyone has the same problem. apple shud be ashamed to be shipping such badly designed buds. these really are the answer.