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With most apps being .99 cents, what's the point? Most apps work similar to your idea, they offer a free but limited or ad-ridden version.
 
if that's the case they should just make all of the app's in 2 versions one as a free demo (witch is only some not all apps) and full version paid apps.

if you like the demo then buy the full version that's how i see that rolling more.
 
I think this has some great potential for GPS Navigation apps. Say you're going on a road trip. Just rent the app for a week or two when you need it
 
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