These headphones are great for exercising. They stay in place and sound great. The cord length is perfect for me. Build quality seems excellent, especially where the cord attaches to the phones. This is important for frequent, rough handling. These phones are not bad looking, either.
A few people complain that the mic doesn't work with their Blackberry, Macbook, Droid, or non-iOS device. The product description clearly states that the mic is designed mainly for iPhones, recent models of iPod touch, and iPads, but will also work with 2009 and later Macbooks/Macbook Pros. If you are buying this for the mic to use with your non-iOS device, check Sennheiser's website to see whether it'll work with your device.
Sennheiser's 680 line includes several different models that have the same sound quality and speaker components, but different ways of making them sit on your ears. This model, the PMX, is one piece that wraps around the back of the head. Other models in this line are called CX, OMX, and MX. They attach to your ears with clips or "ear fins", depending on the model. Also, some models have noise isolation (for when you don't want to hear the annoying music playing over the gym's loudspeakers) while other models in this series are more open (so you can hear honking cars while you're jogging on the side of the street). The PMX is one of the more open ones, but when I'm using them at the gym, the outside noise doesn't bleed in too much.
Also, note that some models have the number "680i" while others are just "680". The difference is whether it has a microphone. "680" (with no "i") means no mic, but there is an in-line volume control. "680i" has the in-line volume control and the built-in mic.
Unfortunately, most of us cannot try these on before purchasing to determine which format is most comfortable. So if you're thinking of buying these, purchase from a reputable vendor that stands by their return policy.
Lastly, I'd suggest comparing the prices of various sellers. I bought mine from OnSale, an Amazon third-party vendor, and paid a lot less than Amazon is currently charging. I've bought several things from OnSale through Amazon's website, and their service has always been excellent.
In sum, the Sennheiser 680 line offers very good sound quality for the money, good build quality, and different models with different options regarding fit, noise isolation, and mic. One of the models in this line is likely to be a great match for your needs. For me, it's this one, the PMX 680i.