I have one on my boat in Greece and find it very useful for medium-term forecast indications. If I was sailing in UK, where good weather forecasts are easily obtained, I would not buy one. A Navtex is excellent for 1 - 2 day forecasts throughout Europe.
I did once look at the Weatherman, IIRC, it depends on a German weather station. The western side of the UK, where I sail was not covered at all well.
That was some time ago, don't know if it's now different.
I have a NASA Weatherman on my boat. As earlybird says it receives data from the German weather office and covers most parts of Europe, some areas are a bit vague. I too find it very useful especially when you are offshore and out of range of mobil internet.
Off Portugal the Weatherman's info of a coming storm (6-7 hours warning) made me run for shelter - the first autumn storm lastet 4 days with waves reported to be up to 9m high further out, was I happy to have read my Weatherman!