I just used it for an hour-long overseas conference-call -- the kind of thing that used to cost me £20 or £30 -- and the total cost was £0.51! I hate how the mobile carriers gouge on long-distance, so I get a grim feeling of satisfaction knowing that I'm depriving Orange UK of the gigantic sums it used to charge me for staying in touch with people elsewhere (of course, I'm also pissed at Orange because an operator named Colin boneheadedly kept on insisting that I'd somehow used 37GB of bandwidth last month with my phone's 3G networking -- something that would require me to watch YouTube 24/7 every day of the month to even approach!).
I can't see any reason why this app wouldn't work when I'm roaming in other countries (I have a T-Mobile SIM I use when I'm in the US) -- I'd just have to drop a buck or two on a local calling-card to work with it.
I write books. My latest is a YA science fiction novel called Homeland (it's the sequel to Little Brother). More books: Rapture of the Nerds (a novel, with Charlie Stross); With a Little Help (short stories); and The Great Big Beautiful Tomorrow (novella and nonfic). I speak all over the place and I tweet and tumble, too.