Sennheiser says these have a "vertical transducer system." They appear to be referring to the fact that the plugs are upright. As I've mentioned elsewhere, that's like calling a book a "horizontal cellulose system." Innovative!
I use these with an iPod Shuffle, been doing so for a few months. These phones will stay jammed in your ears while you're jogging, that's what the behind-the-neck band is for. So far, sweat has not been an issue (though I've never damaged earphones with sweat). Whether you find them comfortable is an individual matter. There's no way to regulate the pressure. I had pain in my right ear, but the right ear bits have now become desensitized and I can wear them for about 45 minutes.
The phones have orangey bits of plastic on them, and if your're counting on that for visibility, you must be jogging in urban camouflage. There is a reflector strip on the neckband. If you're running in conditions where this is actually useful, you should A) be wearing a proper reflector vest, and B) not be plugged into music. Some very cynical gimmickry by Sennheiser there.
When the phones are broken in, the sound is pretty good. Treble is bright and detailed, bass is reasonably defined, but the sound as a whole feels a bit closed-in and wooly. No doubt this is partly due to the closed-back design.
Things I really appreciate about these phones is the single cord (though it's too long for my setup), and that they seem quite sturdy; and of course that they stay on my ears during all kinds of exercise (and now they don't hurt so much). The cable clip is a clever idea but the plastic is bittle; I used a thin screwdriver to extremely gently pry it so that I could get the cable in.
If your workouts involve a good deal of lying on your back with your head on the floor or bench, these won't work. They also don't work if you wear clothes with a high, stiff collar.
My gold standard for exercise phones is still the Koss KSC35, earclip phones with the same elements as the PortaPro. Four years on they still sound terriffic. These Sennheisers are a different concept, with some clever solutions, and maybe the compromises will work for you.