Rob Napier, the author of iOS 5 Programming Pushing the Limits, mentioned there are several models of selling apps on the App Store:

1. Write an app and sell it
2. Publish a free and a full version
3. Ad supported by third party or by iAd
4. In App purchase

Surprisingly, the author said that the most workable model is (1) in terms of sales. I would think that (2) with fairly limiting ability for the free version can bring more sales, as people without trying, might not plunge down $0.99 or $1.99 for something they haven't tried? I for one, might not have purchased Angry Birds if I didn't try their free version first. Also, I think it also depends on the situation: if the app is about alarm clock, and there are already 5 alarm clocks in App Store that are free, then your app that is $0.99 might not be that eagerly purchased. If yours is also free, and users really like it out of all the other ones, then they may think, $0.99 is nothing to get a good alarm clock, and gladly pay you the $0.99 in exchange for a full version of the alarm clock, something that they can't get with the free version. (such as the full version can let you choose a song from your Music Library for the alarm).

Could (1) work only if the user definitely want it and have no substitute? How might it work the best?