UPDATE: In December 2012, the Google Maps app was released in the App Store, so you should now install the free app instead of using the instructions below, though they still work.
With many iOS 6 users dissatified with Apple's new vector-based maps, it can be frustrating that there's no way to go back to the trusty old Google Maps app. See also: Apple says iOS 6 Maps isn't finished
Although you can't download a Google Maps app from Apple's App Store, you can still create a shortcut icon on your iPhone's Home Screen which gives you quick access to the familiar Google Maps website. Here's how to do it.
Launch Safari and browse to Google Maps - maps.google.co.uk (or simply search 'Google Maps').
When the site is loading, you might be asked to allow it access to your location.
You should do this if you want it to pinpoint your location on the map, which you probably do. Of course, you can deny access if you prefer, and you'll still be able to browse Google Maps and get walking and driving directions.
When Google Maps loads, tap the share button at the bottom of the screen, (the rectangle with an arrow pointing out of it) and you'll see the screen below:
Tap Add to Home Screen, and name the shortcut 'Google Maps'.
Now you can tap the icon on your Home Screen to get straight to Google Maps.
The web version of Google Maps offers most of the features of the iOS 5 app, including Google search for places, directions, pinch to zoom. However, you don't get Street View and the interface isn't quite as slick. For most people, that's a small price to pay to have accurate maps, until such a time as Apple sorts out all the problems.
Also see: iPhone 5 Advisor