It depends. Normally Dems have an advantage that is slight to significant but not huge. Really, the best comparison would be to what the early voting looked like in 2004 and 2008 compared to this.
In 2008, many of us who expected McCain to win or thought the race was too close to call ignored reports that Dems were way up in early voting in swing states. The good news is that this year we are seeing Republican advantage in swing states, most notably Ohio and NC. LS can probably elaborate, or you can just look at his in forum page.