The software has never given me any problems. There is an option of choosing guitar, bass, violin, banjo, and other stringed instruments, in order to highlight the specific frequencies that are necessary. I've never had a problem with latency or other issues, and the built-in Iphone mic is surprisingly capable of handling this feat. This is assuming that the room you are in isn't loud, which is a problem for any built-in microphone based tuner.
While this is a pretty awesome application, I think that it's basically a little too late for most musicians to settle for it. For one thing, using it in a loud room or a gig situation is a pain, simply because the built-in mic is too weak to pick up the instruments. If you are going to use it simply at home, with an acoustic instrument (or a plugged in electric one), it's probably worth it. Especially if you lose the tuner you already had, which is what happened to me. Overall, this is a surprisingly useful Iphone app for those moments where finding a tuner is going to be more trouble than it's worth.