I have tested it :) I've asked it to translate English fictional text into Japanese, it falls over often. It's unnatural and often makes no sense at all. It doesn't compare to a typical professional translation (which are often not that idiomatic either), let alone a really good one.

I'm sure it'll be doing that in five years, but not now.

One interesting thing is that's it's nondeterministic, so sometimes 'For chrissakes' turns to ちくしょう (Damn!) but sometimes to クリスのために (for Chris' sake). Sometimes 'the goddamn door' turns into クソドア ('shit door'), sometimes the goddamn changes the phrasing of the whole sentence instead. If you run it five times and take the best sentences out of all five runs it's probably quite good. Maybe prompting would help too, I said "idiomatic Japanese" but it still usually translated it in a very "foreigner Japanese" way typical of US drama/movie translations.