Like most iphone apps, it will be a thing you think is pretty cool that you try once, enjoy, show all your friends, enjoy their gleeful cries, and then forget about. You spend $2 on it so you got your money's worth, the dev makes a ton of money, and everyone is happy.
I don't think anyone would seriously consider this over a mouse/keyboard, as it would be replacing the keyboard and you cannot cram nearly as much functionality, with nearly as much rapid accessibility, onto that little screen as good ol' qwerty here. It is really neat, though.