Apple's policy specifically states that you have a 14 day return policy if you are not satisfied with your product. I think if you genuinely think that, if the product reaches some realistic expectation of yours, you'd happily continue to own it and fully pay for it, then it is morally ok to buy it knowing that if it doesn't you'll return it. But if you are squarely in the "I can absolutely not afford this but it would be fun to play with it for free on their dime for a couple weeks", then I'd argue that it is morally very questionable. But, having said that, I can assure you their pricing and sales model has built in those considerations as well, so there's that perspective, too.