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Oh how I wish Apple's App Store had this feature:

The Android Market business and program policies also include an item that says users can return any application for a full refund within 24 hours of the time of purchase. In the absence of a trial version of applications, this offer will let users return an application that might not deliver exactly what they expected. [Source]

I've held off buying software (for example) because there's no demo and I'm not sure it's worth $9.99 or whatever. Additionally, this feature would free developers from having to create a "free" version to serve as the demo.