CNET has a review of the Cardo S640 and writes - “The Cardo S640 is a small and compact device, only measuring 1.5 inches long, 0.7 inch wide, and 0.8 inch deep and decked out in a simple, glossy black. As we mentioned, the Cardo S640 is more of a Bluetooth clip than a headset–the entire device is the clip itself, with a wired earbud attached to the top. There’s a volume button and an LED on the front of the S640, while the multifunction control button is located on the earbud. While we found the buttons pretty easy to press, we weren’t too pleased with the concept of the volume button–instead of having a volume rocker that you can use to increase or decrease the volume, you have to cycle through the volume settings from lowest to highest with every tap. The wired earbud is a pretty standard size and fits comfortably in the ear. The S640 even comes with two earbud foam covers for additional comfort.”

Read more about the Cardo S640 Bluetooth headset

Cardo S640 Bluetooth headset Review at CNET