Good MONEY magazine article showing how little redemption value one gets from Delta miles compared to other airlines. Consider Delta miles to be worth about a quarter (.25 cents) to other airlines miles. That could impact your choice of airline, if you're considering miles value.

Data from MONEY magazine article:

"Likelihood of getting a seat on a selected route"

CO: 97% (dom) / 46% (intl)
UAL: 81% (dom) / 56% (intl)
AA: 66% (dom) / 50% (intl)
US: 10% (dom) / 11% (intl)
DL: 19% (dom) / 7% (intl)