Here's my highly recommended route from Ashland to Rohnert Park when you want to get here in a hurry but want an interesting, twisty drive while avoiding the I-80 traffic you'll get when going the recommended yahoo route:
http://maps.yahoo.com/#mvt=m&q2=lower+...5548&mag=11I've gone this route twice in as many months.
If you had time to kill, then I'd recommended going from Ashland down I-5, crossing in at 96, and then going 101 from Eureka all the way to Rohnert Park. Awesome drive, but it takes an extra day to really enjoy it.