I just initiated my 3rd exchange of the Retina Macbook Pro. My first two rMBP's had cosmetic defects on the aluminum exterior and the last one had significant screen burn-in. All the rMBP's I've had so far had screen burn in to varying degrees, the last one was by far the worst however. I appreciate that Apple has exchanged these items relatively trouble free but I'm wondering just how many chances I should give them? If you've had an exchange, how many exchanges has it taken to get the perfect machine?