My iPhone's always on me, I am required to carry it for my line of occupation. That makes it incredibly handy to always have a tuner with me. Just the built in mic works fairly well, too. I have tuned up many of out of tune guitars at GC, on the spot, just because it was with me. I have the TC as well and it works great.
I find more and more uses for it as a handy tool. The voice recorder works great for a quick acoustc demo when inspiration hits and you are afraid you are going to forget the song if you don't get something down.