It was obviously abhorrent and still criticized 70 years later (which it should always be), but was there proof that it was successful and beneficial to national security? Were any spies convicted or captured because of it?
Or was it overwhelmingly useless?
The very fact that the majority (but not all) of Japanese Americans in Hawaii were not interned, and yet that there was no sabotage in Hawaii, should already say something as to the effectiveness of this policy. If anything, it should be noted that areas that had pre-existing grievances against Asian Americans and the Japanese in particular were the areas of focus for internment, specifically California. There, pre-existing economic and cultural conflicts resulted in a call by many Americans to intern the Japanese (which has the side-effect of forcing the Japanese-Americans to sell off their assets at fire-sale prices and reducing competition).
It should also be noted that one case of Japanese espionage, Takeo Yoshikawa, was a spy in Hawaii that kept notes on the Pacific Fleet prior to the Pearl Harbor attack. For what it was worth, he did not trust the large Japanese American populace in Hawaii as he thought they were more loyal to America than to Japan. As evidence later showed (with the Nisei volunteers), he was right.
Or was it overwhelmingly useless?
I wouldn't say it was "useless" just not well thought out. There was a hysteria after Pearl Harbor that was real. Moves such as the internment of Japanese Americans were a reaction to that.
On Thanksgiving, a friend of mines mother came. She is late 80s and lived through WW2. Somehow the conversation turned to WW2 and I mentioned that there were large anti-aircraft emplacements all over Southern California - the ones in Huntington beach were now buried under mounds of dirt.
At this she lit up and started describing "the Battle of Los Angeles" insisting she saw 3 Japanese bombers that night. She vividly remember the sirens, anti-aircraft fire and searchlights.
Knowing something about WW2, I discounted her and told her that no Japanese bombers ever bombed the West Coast of the US (Alaska excepted). Perhaps she saw 3 off course USA planes...but she was unconvinced.
She then recounted how there were Japanese Americans spies all over Southern California. Ones near where she lived had planted crops in an arrow formation to direct Japanese bombers to a nearby airfield (I think it was Lockheed back then).
Not wanting to get into an argument with an octogenarian of the "Greatest Generation", I dropped the subject. But it gave me a real insight into the kind of hysteria that must have been present back in 1941 Southern California.