I'll follow this with rapt interest, as I've asked the same.

A nephew showed us his Iphone with this feature at my request. I spoke a sentence into it, and it mis-translated it. But I knew that, because it also printed the English as well as the French which it thought I'd said. I said it again, more into the bottom of the I-phone than into the screen as I'd done the first time, and it understood me 100%.

It spoke the French faster than I even think in English, so it might lead the unwitting target to jabber quicker and more at length than the I-phone might be able to handle. In other words, it finished saying "where is the nearest bathroom" in French in about as much time as I'd take to say "where is" in English.

I could see where it could be helpful in a small rural village, though.

I'm curious if I could rent such a phone for two weeks.

Also, does the device (I-phone OR Android) have the ability to learn one's speech patterns, so that mis-translations occur less frequently as time goes on?

I've never used one and never would. I can't even begin to imagine how bizarre it would be to try to have a "conversation" with someone using one. I mean, what do you do - ask the other person to wait while you type in your question, then hand them the device? Then what happens? The other person types in a response and hands it back to you? And then you repeat this awkward process until the "conversation" is over? Seems really weird to me. I can't imagine any normal French or Italian person wanting to engage in that activity. Besides, electronic "translators," like internet "translators," are notoriously faulty.

It makes more sense to learn some French and Italian.

I agree with StCirq-
I know they have improved since I used one about ten years ago. Unless you go exploring outside the tourist area then it is of little use to you.
I found out that I do better with a picture translator booklet then anything else.

I have used one in France and loved it. But in only special circumstances. For example, knowing I wanted to go to a store to buy aspirin, I managed to figure out what I wanted to say before I got in there. It helped me quite a bit. Though again more in preparing what I wanted to say than during a conversation.

Did the women who want to buy a translator say what they wanted it for? If you're going to normal tourist type areas people will speak English. You may use it for a sentence or two but you can't have a real conversation using a translator.

Imagine the reverse situation where someone from Japan is visiting you and you communicate through this device. Imagine how tedious it would be.

If you pull out one of these things in Paris people will disdainfully tell you they speak English so no need for a translator.

There's a lot of France beyeond Paris.

One doesn't type in the I-phone; one speaks into it.
Likewise the other party.

If the other party "disdainfully" said, "I speak English, you dolt, don't demean me with your stupid machine.", I would have successfully initiated conversation with him/her, which is what my goal was.

tomboy,

What's the name of the app you tried?