I train for two full marathons per year, and I sweated my way through two sets of ear buds in less than a year, so I was looking for replacements that were specifically designed for the rigors of serious running. I quickly found the Sennheiser/Addidas branded models online. I chose the MX680 "ear fin" models, as I did not want a behind-the-neck band flopping around while I run, and I did not want the isolation of a traditional sealed ear canal ear bud, since I prefer to hear cars before I am under them.
The MX680 is very close to perfect for this combination of criteria. The ear fins feel strange at first, mostly because it feels like they cannot possibly stay in place, but they do, and they are comfortable. They're so light I can hardly feel that they're there, and I can hear ambient noise, so the overall design works as advertised. And there are plenty of sizes of fins and ear cushions included to fit every size and shape. The MX680 does not produce the thunderous bass of expensive, tightly sealed ear buds, but overall sound quality is very good, including decent bass (much better, for example, than the tinny sound that results when sealed ear buds slip loose in the ear). And the open air sound is great for running outdoors.
I purchased the iPhone compatible MX680i, hoping that at least the mic would work for phone calls with my Motorola Droid. It doesn't. The remote partially works with the Droid, allowing me to pause or advance tracks, but volume and mic do not work. I knew I was taking a risk that this would turn out to be the case, but for others trying to choose between the MX680 and the MX680i, don't spend the extra money if you are not going to use it with an iPhone. The MX680i **will** work as ordinary headphones on any 1/8" stereo jack, though. In addition to dozens of miles of all weather outdoor running so far in the three weeks I have had these, I have also been happy using these on hotel treadmills and on my notebook computer when I did not want the isolation of my more expensive ear buds.