Swing It for the Mac is indeed a nice program and fairly cheap.
On the iPhone/iPad I have used several different programmes (iSwing, V1 Golf, Swing Plane and Swing Reader).
I haven't tried the 'Digital Mirror Mobile' app from JC Video yet bur from my experience with the other apps I think SwingReader comes out on top (there are a few things that are better in V1 Golf but for most parts SwingReader is better). Especially the commenting on frames and export feature is nice.