I was going to give you my version of answers to your questions but http://en.wikipedia.org/wiki/Navtex
probably(?) does it better!
Yes, it is useful - gives the same info as the CG broadcasts and continued to work while they were working to rule, Doesn't always get a signal in the west coast, is updated about the same time as the new forecast, roughly 0600, 1300 and gives gale warnings when they are issued. I certainly don't rely on it - watching the sky and the barometer remains No1, but it's nice to have.
It also includes nav warnings, subfax, gunfax and all the other bits and bobs all (de)selectable at will and peculiarly, seems to be better at picking up Dutch, German, Norwegian, French, in fact almost anywhere than the UK stations but again (de)selectable.