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3 great articles on Apple:

Understanding Apple requires an analysis of Fundamentals and Psychology
Apple has created 3 groundbreaking products over the last 10 years and in the process have created a moat for the company - huge economies of scale, unmatched buying power, ecosystem of products, and a brand associated with quality.
http://www.institutionalinvestor.com/blogarticle/3151025/Blog/Understanding-Apple-Requires-an-Analysis-of-Fundamentals-and-Psychology.html

Why David Einhorn Is Wrong About Apple
Ecosystems are overrated (switching costs are low), every product company disaster has had superfans, the moat is narrow.
http://beta.fool.com/joekurtz/2013/01/22/why-david-einhorn-wrong-about-apple/21906/?source=eogyholnk0000001#.UQaMNoHM-p0.twitter

Apple Versus the Strategy Professorshttp://blogs.hbr.org/fox/2013/01/apple-versus-the-strategy-prof.html