I have an Sony Xperia Neo V And it works fine. But is not a 100% reliable, in video mode, when monitoring without recording it works very good, even with the filters working (have a pretty good ones) and without lag.. BUT, in the moment I press the record button, the speed slows down.. there is some lag, and sometimes (very oftern) the speed slows down a lot, and even it freezes completly. Oh, and when recording I can't use filters at all, that makes the app to freeze.
I guess that it's for my smartphone. It's not the best. A newer and more powerful maybe works better. And would be great.
It doesn't shut down the main display, wich is awsome. And you can transfer the signal trough wifi to another smartphone or tablet.
I have even connected via hdmi to an LCD TV, and have three monitors working at the same time

With a powerful device, I think it is an very useful tool and must have.