A few weeks before SXSW, I installed Dark Sky on my phone, and I've been consistently impressed since then. Dark Sky accurately predicted rain in Austin and San Francisco, and even manages to give useful forcasts in San Francisco's microclimates. Because of SF's coastal mountains and fog, it's regularly 50F by the ocean and 85F if you go 15 minutes inland. Even in this hostile environment, Dark Sky gives accurate predictions by comparing your physical location with real-time radar and temperature data from Forecast. Because the app does most of the hard work on servers and sends you a push notification when you're about to get rained on, I haven't even noticed any impact on my battery life.
The upshot is that if Dark Sky lets me know it's about to rain, I reach for my rain coat. That is well worth the $4 price of entry to me.