In reality, I think we won't have a complete picture until traffic patterns resettle, as people discover that free alternate routes aren't actually faster, but let's take today's commute as a starting-point litmus test.
Here's what the Seattle Times had to say about this morning's traffic:
About 37 percent fewer drivers than normal crossed the 520 bridge between 6 and 7 a.m. this morning, transportation officials said. ...
Officials were closely watching other major roads, including Interstate 90, to see how they would be impacted by drivers changing their routines to avoid the tolls.
But early this morning, there were no reports of significant delays.
Overall, the typical morning commute on parallel (and toll-free) I-90 increased an average of 4 minutes, the Bellevue Reporter reports.
Although a King County Metro spokeswoman says the agency won't have ridership data for its cross-lake routes until the end of the week, anecdotal evidence suggests that more people are riding across the bridge (and enjoying a faster commute). I also have a call out to Sound Transit for ridership stats on their bus routes across 520.