If you lock your phone while in navigation mode, Siri will continue to give directions and you'll be able to see your current location and any other upcoming direction information on the lock screen.
There are a lot of nice touches to navigation mode. For example, I like the floating signs that identify nearby streets, and the fact that the streets you're going to actually turn onto are marked in blue rather than in green. The instructions to make turns and such are generally large and easy to read, which is good, since you don't want to spend time squinting at tiny text while you're driving.
The directions themselves are a mixed bag. I live outside of Boston, which is a notoriously hard city to map; it's the rare intersection that has more than one ninety degree angle in it. It's also a city with a high degree of odd and perplexing streets, plenty of one-ways, and crazy intersections. If London cabbies are required to prove that they have "the knowledge" before becoming officially licensed, let's just say that many Boston residents have something we could dub "the know-how." Maps does reasonably well in my city, with logically straightforward routes--but many of them are ones I wouldn't take.
That said, the primary use of a navigation device--for me and, I'm sure, many of you--is to direct you when you don't know which way you should go. In that sense, iOS 6's turn-by-turn directions are generally sufficient--but it probably wouldn't hurt to have some other maps on hand, just in case.
For what it's worth, you can still get the old-fashioned step-by-step style of directions, with this handy little trick: When you ask for directions, enter something other than your current location in the starting location field. (It can even be your current address--just don't use the current location.) You'll get the same street-sign-style directions, but you can swipe through them, and Maps will show you were your turns are.
Walking directions use that same step-by-step style rather than turn-by-turn; other than the styling, you won't find much difference from iOS 5 Maps.
Transit directions, however, are a whole new ballgame, as Google apparently got the public transportation database in the divorce. Instead of including built-in transit directions, Apple has added an API to let third-party developers accept transit direction start and end points.
So, while there's still a toggle button for transit directions while you're on the Directions screen, selecting it and tapping Route will produce a new screen labelled Routing Apps. There are two separate listings on that screen: the first shows you any apps currently installed on your device that can handle the specified directions along with a Route button; the second shows you apps available on the App Store that can do so.
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