Hitherto there weren't many competitors that could match, or even compete with, Amazon's stock and delivery speed. Walmart is close, as is Costco, but Amazon is where it's at.
For quite some time Prime was... Not a good value for us. Then we got Video and Music, to a certain extent, and it's a fair value if the content you want is there.
But the problem, and why myself and many peers have cancelled prime and started shopping elsewhere, is that the basis of trust in the quality of their stock has fallen away completely. The amount of garbage with four stars or more is scandalous, and the likelihood of receiving a knock off product is too high. Opening a box to find a product similar to but different from what you intended to buy is all too common.
Amazon, get your shit together before you irreparably harm your brand.