As a recording engineer, I know and respect the Sennheiser name. Their microphones and headphones are staples in most studios. And while adidas isn't the first name you think of in the sporting equipment world, their products are known and respected in many venues.
The two companies are working together to produce a line of earphones designed for the athlete. There are four models: two earbuds, one over-ear earclip, and one neckband style. All four are water-resistant, sweat-resistant, and the Kevlar cable is able to withstand temperature extremes. They also have inline volume controls that are easy to reach during a workout. The frequency response for all four is 18 - 20,000 Hz, with a maximum power handling of 120 dB. Be careful with that.
I had a chance to wear the CX-680 earbuds during my workout today. The "EarFin" design looked like a gimmick at first glance, but after locking the fin in place, the CX-680s were secure, even during an active spin class. More than anything though, the sound was superb. Because of the good, secure fit, the bass response was really tight and with outside noise blocked out, the high-end detail and clarity were sharp.
Two major players teamed up, and the resulting earphones are definitely winners.