They look kind of useful but I'd imagine you'd get pretty bored with it after a week or so. My big problem with these devices is that most of us have our phones with us 24/7 anyway and it's not that much of a hardship to pull my phone from my pocket.
If you could leave your phone at home but still access and make calls from it remotely (miles not meters) via a watch then that would be useful IMO. I guess we are a way off that yet though.