Heard of but never looked into enough to dig into the math. I'm not sure it's that interesting but could work out if there's a guaranteed redemption ahead. Note the tax implications here - your marginal rates could have an impact on this: "For tax purposes, miles are considered to have a value of 0.42 cents. At our current rate of 2.5 miles per dollar saved annually, this represents a taxable APY of 1.05%." Just to make my brain feel better at at around a 35% marginal rate with $50k in for a year you'd net 125k miles - $183.75 ($525 * 35% marginal tax rate). A bank account at 5% effective APY for the same tax rate would yield $1625 ($2500 * $875 at 35% marginal tax rate).

Effectively "buying" 125k AA miles for $1808.75 (opportunity cost $1625 + tax cost $183.75) or 1.45 cpm. I'm not too up on AA miles, but possible to buy them more cheaply than that? That said if there's a guaranteed high value redemption then seems easy enough to have a bit more fun with an emergency fund.