I'm really excited about starting AMS in the UK. I'm wary about other countries outside UK, Australia and Canada because I don't have many sales through Amazon outside those three countries. My home is in the US.
The other thing to consider is review count. Surely people click on your book through AMS when looking at the # of reviews and many of us just don't have a lot of reviews, or any at all, in some of these outlying countries. Perhaps lower competition will offset this? I don't know. Even in the UK, where I sell a decent amount of books, I only have one review posted. But I'm giving the UK a shot.
Anyway, it's still great that we have the ability to do this now. They'll probably push the functionality everywhere internationally soon.