Recently, I released an article noting that early voting correlates at about 0.4 with final voting. This data refutes people who say that we should totally ignore early voting numbers. However, I should have also noted that early voting tends to overpredict democrat performance.
Here is data on the same 12 states that 538 used in their analysis of early voting.
On average, early voting indicated that for every 100 republican voters there would be 134 democrat voters. On election day, for every 100 republican voters there were actually only 105 democrat voters. Early voting is biased in favor of democrats. Keep that in mind when looking at early voting numbers.