Go for it
MrBo,
If you live in the UK, definitely go for it. I still (sort of) live in MAN and signed up for the hell of it last January to attend a friend's wedding in Miami. I ended up travelling a lot more that year than I would ever have guessed and made Gold (purely on leisure flights) and even ended up with a wife (although that's not one of the stated benefits of the Dividend Miles scheme!)
I got a bonus for activating the account, but it was not 5000 miles. However, you should get an additional 25% mileage bonus for being a European flier. Once I get set up fully in my new life (i.e. job etc.) I'll tell US that I now live in LA and not Manchester.
('twas an easy choice: Manchester Ship Canal and River Irwell or Pacific Ocean)