The app, from Presselite, combines your GPS with an up-to-date registry of stations and schedules. This means you can get directions for the fastest route, without worrying about which lines are running normally (because they never do). It also updates according to local service changes, so you won't get trapped on an express train to nowhere.
'New York Subway' works seamlessly with Bing and the Metro UI to give you a seamless interface. Especially when the MTA is shutting down routes and seemingly doing constant construction, you'll appreciate a reliable guide. Sure, you could just trust Google Maps, or check the MTA website, but isn't this easier? At $0.99, we certainly think so.
source: Zune/Windows Phone Marketplace via WPCentral