If you want just simple radar (which is usually what you need while on the go) I use a very simple app called RadarNow! that just has composite radar and nothing else, so the data demand is low and the app efficiency is high. For forecasting many other apps work reasonably well. I’ve heard a lot of good stuff about Dark Sky but I have not particularly used it.
With my phone in the cradle on my bars I can usually manage to hit the icon for radar and then I can see if that dark up ahead is overcast, or real rain.