I've got it (and like it).
The basic box doesn't require any subscription but it only gives you things like BBC iPlayer, ITV Player, Youtube, etc. That's fine for people who simply want to make a 'non-smart' TV into a 'smart' one but otherwise you need to buy one or more passes.
A Sky Cinema Pass costs £9.99 per month and gets you access to over 1000 films.
An Entertainment Pass costs £6.99 per month and lets you watch Sky 1, Sky Arts, Sky Living, Sky Atlantic, Gold, Fox, ITV Encore, Comedy Central, MTV, ABC Studios, Nat Geo Wild and the Discovery Channel.
(There's a separate pass available, at £2.99 per month, for access to 6 kids' channels)
You can also watch all of the Sky Sports channels (except pay-per-view events, such as world title boxing matches) by buying passes but, while they're useful for occasional viewing, they work out expensive for regular access. A single day costs £6.99. 7 days costs £10.99. A month costs £33.99.
I've just got the Entertainment pass, which I find useful for watching Gold (and a few programmes on Sky Arts, etc). The good thing about it is that's there's no commitment. I can cancel my subscription at any time (and then pick it up again later if I wanted to).
As Gromit says, there are 'starter packs' available in many larger supermarkets, which work out cheaper than buying the box and your initial passes separately. (They're also available online but they're sometimes a few pounds cheaper in supermarkets).