Luke Wroblewski explains why those overlay introductory “tour” screens you often see in apps is such a bad idea in Designing for real world mobile use:

These issues stem from the fact that introductory tours show up before you ever get a chance to use an application. Most people are eager to jump right in and as a result, they skip reading the manual. The ones that do read haven’t seen the interface yet so they don’t have any sense of where and how the tips they’re learning will apply.

I completely agree, and have written about this before in Best practices for user onboarding on mobile touchscreen applications, where I give three guidelines for app onboarding:

• Make sure that users have familiarized themselves enough with your app to have the correct mental model before you start teaching them how to use it.
• Show users only the information they need to take the immediate next step(s) for using the application.
• Make sure users have a clear link between the information you give them and how to access/use that information in their everyday use of the app.