The S450-XT and the Plantronics Voyager Focus UC both have a very specific market: people who want stereo music, but also need crystal clear call quality in a range of environments. For those folks, a standard mono Bluetooth headset or a pair of stereo Bluetooth headphones just won't do.
Long battery life; Wired and wireless options; Excellent outbound noise cancellation
Bass-heavy at the expense of midrange; Bluetooth quality isn't great if connected phone isn't out in the open