If you're doing the counter commute, Kirkland->Seattle Business District in the morning, 520 is a great option. Most people have abandoned it due to the toll, but the only area where things get sticky is the merge from 405->520 where you need to merge left twice in short order and then merge left again directly after the Burgermaster exit. The merge across I5 to the Union exit is annoying since the backup for the Mercer exit (and people that never realized the needed to take Mercer) extends almost to 520, but if you wait in the third lane from left until you pass Mercer, you can usually merge gracefully right and take Union.
The return commute is only screwy when there's a big con - SakuraCon for example.
I did take Mercer for the three months I worked at Lab 126 in Fiona, but I assume it's better now that construction is over. It was much worse.