As an avid customer and advisor for brands selling on Amazon, I continue to be amazed at what the Prime program has become and is becoming:

Captive: Less than 1% of Prime customers are likely to consider other mass-market retail sites (e.g. Walmart.com or Target.com) in the same online session.
High Volume: Prime members spend $1,500 annually vs. $625 of typical Amazon customers.
High Converters: 63% of Prime members carried out a paid transaction in the same visit, non-prime was 13% vs. Walmart.com at 5%.
Growing: By 2020, analysts predict 50% of American households will be Prime members.

Prime's unapologetic pursuit of simple commerce is clearly ahead of other rivals. Features like one-click checkout, pressing Amazon Dash buttons for replenishment and Prime Now same-day/same-hour shipping continue to lengthen their lead.