OT - Gleno, it depends what time you go. What sucks about I-5 is you go from 4 lanes northbound and 4 lanes southbound to 2 lanes north and 2 lanes south as you approach the city and people from the ramp on either sides of the 2 lanes are trying to get all the way to the other side to get on a ramp. It's just a cluster-f... and poor road planning. It's like that for about 5 miles as you wind through downtown Seattle. If you get close to Seattle around 10:30/11ish usually traffic is a lot better and actually moves.
If you go around 405 during rush hour around renton it can be pretty bad too. A lot of people live that way cause they can't afford housing around Seattle or in Kirkland/Bellevue due to the IT folks working at Amazon, Microsoft, Clearwire and T-Mobile. Then going back over to I-5 over the floating 520 bridge is pretty bad too as everyone is trying to get over to downtown Seattle or Redmond/Bellevue from Seattle.
In general it's bad around Seattle during rush hour but that can be said of any city. If I were you I would leave Portland later so you don't have to deal with the traffic crossing over to WA, and then by the time you get to Seattle it'll be 12 and traffic won't be bad at all.