The interface is very nice—much better than Google Translate—but the translation in my tests was almost always mistaken. I used English to Portuguese for my tests, since I can normally detect errors in translations into Portuguese. I got results ranging from bizarre to the opposite of what I said to accurate to obscene. I tried to avoid trivial sentences like "hi, how are you?" since any conversation that matters will quickly become much more complex than that.
Here are some examples:
"Gabriel I killed a fly" first came out as "Broke that father"—completely unrelated and not even in the right language. I tried it again and got the correct result: "Gabriel eu matei uma mosca".
"I studied Linguistics at the University of Michigan" was not correctly recognized, but instead was interpreted as an obscenity.
The price is very high for casual use. If you are traveling in another country, then the price is trivially low, since this could make an enormous difference in what you are able to do while you travel.
At the same time, you are risking the translation not simply being wrong, but saying exactly the opposite of what you intended. And you are risking that it says something offensive before you have the opportunity to correct it. The emotional (and therefore relational, and possibly therefore financial) damage will be done upon hearing. Explaining that you didn't mean to say that may only partially ameliorate the situation.
I recommend using this only after you alert your interlocutor to the fact that the translation is OFTEN wrong, and only if your ability in the foreign language is high enough that you can recognize when it has made mistakes.
*Reply to Developer:*
You are probably right, but are you mentioning that because that is an Apple-provided function, rather than a part of your app?
In order to deal with the very real problem of inaccurate voice recognition, you could display (or play as audio) your interpretation of what the user said, and allow a moment for the user to confirm or deny (and then correct) that interpretation. This is similar to what Siri does, printing your words on the screen as you talk and allowing you to edit if necessary. I'm sure you can come up with a nice way to make the UI function since your company clearly has good UX design skills. The confirmation could be an optional step, in case some users find it unnecessary.