I used to use Air Nav Pro on my iPad / iPhone but as the developers added more features, it become slower and slower until the program would often crash when opening AIPs. However what really caused me to consign this program to the bin was the fact that, sporadically, the programmers would release an update which would then delete ALL your maps.
Ok, the maps weren't deleted, they were still on the iPhone / iPad BUT the links to them were gone and the only way to have the maps working again was by reinstalling all the maps. In one case, I was abroad and had to download 3GB of map data from their store because of their abject incompetence - 4 times this happened on various releases (never subsequent, always a few months apart) until I said enough.
From what I understand, the software is not a legal as the sole means of navigation, basically because the program / hardware has never been qualified for aviation use, however it works reasonably well if you have the maps / aips downloaded; also, the position of the unit in the plane will determine how accurate the software is - after all, if you have a high wing plane and the iPhone is mounted low in the cockpit, the chances are, it will lose the signal. I used to use an external GPS receiver but in the Archer, mounted to the yoke, the iPhone never lost a signal.
To conclude: software is ok, if only the programmers would stop issuing releases which then require you to reinstall your maps..... Legal? Not as the sole means of navigation, but it works well....