You have to put a lot of faith into that software if you actually use it while driving because checking if the answer is correctly chosen with the phone hanging over the wheel is a big no-no in my driver's book (it does beat actually typing out the message with one hand but that is already an even bigger no-no).
Semi-automatic voice activated replies is something I'd like to try out to see how it feels though. I am curious how the experience would work out considering I am more of pushing buttons guy.