Yes, I have done it a number of times and would suggest it if youwant to do some sightseeing on the way up or back. Nancy Norris just did this a couple of months ago, so maybe she will share her experiences.
Flying into Seattle can certainly be cheaper and renting a car is by far more time efficient than the SuperShuttle experience. I would definitely recommend this as a very viable, relatively easy and enjoyable option.
Nancy