As you may already know, Google Maps is available for iPhone. It seems to work better than Apple Maps (in most cases).
Here’s how to use Siri to find directions with Apple Maps:
1) Ask for directions as you normally would, but say “in transit.” For instance “Give me directions to Walmart, in transit.”
2) Siri will inform you that she is finding directions. Then a screen like the one below will pop up (note that Google Maps has already been installed on my iPhone. The other apps shown are links to the App Store):
3) Hit the ‘Route’ button and Siri will launch Google Maps. Awesome.