I have a sales droid mockup of one of these on my desk at work. It's swag from a recent wireless conference that I attended. Just too cool. Voice dial. Touch to talk. Caller ID.
The only drawbacks that I could come up with is that this is not a phone. Other people in the room get to hear both sides of the conversation. Also, the size of the battery probably doesn't lend to extended conversations.
Obvious uses? Hospitals and warehouses where the user is normally mobile.