As mentioned, the LG Optimus G comes preloaded with the awesome Google Maps app and the functional but overpriced AT&T Navigator. Between those two you shouldn't be wanting any more navigation options.
Want to make an iPhone 5 user cry? Show them your Google Maps, which has only gotten better with age. Not only does it provide turn-by-turn directions on Android devices, it's even begun to show interiors of famous places and large metropolitan transportation stops. These days it even calculates bike routes and public transportation, where available. Apple Maps has a ton of catching up to do.
And so does AT&T Navigator, which only works as well as a moderately priced GPS devices. It gives accurate, spoken turn-by-turn directions and provides maps that are none too detailed. Still, it can't match the data or the interface of Google's map app, which has become extremely intuitive over the years. It's a wonder why AT&T even bothers including it, let alone charging for spoken directions.
Both navigation services use voice recognition to accept spoken commands, and with this we find one place where Apple still rules the roost, at least when compare to Android 4.0: Ice Cream Sandwich. Post-iOS 6, Siri is still the reigning champion of understanding what you mean and accepting commands in colloquial terms. Voice command on the Optimus G's maps are decently accurate, to the point where it can serve as a minor shortcut, but not as a reliable hands-free device. Asking for directions to a contact in your address book works well, but performing a more general search or complex command produces unreliable results.
This is one place where the upgrade to Android 4.1: Jelly Bean should help, as the latest version of the Google OS has improved voice recognition technology. We'll reevaluate and update this review when the update becomes available.