In my experience, which I'll admit up front is extremely limited, the donation model isn't really that effective. The people who donate are incredibly generous, but also quite rare in terms of overall traffic. This isn't because people don't want to pay for quality material, or rather that they're unwilling to do so, but more because people tend to be a little lazy and put off and eventually forget to donate after reading. I, being one of these, know of what I speak.
It's a nice idea, but except in certain niche areas I don't think it's effective.
Tenhawk AKA Evan Currie
Author, Thermals
http://www.amazon.com/dp/B004URUXKQ?tag=fiwi0f-20
http://www.amazon.co.uk/Thermals-Ans...n-associate-21
https://www.amazon.de/dp/B004URUXKQ?...associate02-21
http://www.smashwords.com/books/view/53324
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