I've actually ridden sections of this route as part of various planned events (mostly by the Salem Bicycle Club) and it is nice and scenic. I personally find the flat farmland to be just a tiny bit dull, though.

If you're looking for more of a challenge and jawdropping scenery, you may want to check out the Three Capes scenic route.

plantluvver, the San Juans are wonderful. It can be pricey but there are some cheaper B&B type places you can stay at. What's also nice is that inter-island ferry rides are free to pedestrians and cyclists. So once you pay the fare to get onto one island, you can island hop and explore the other islands by bike. On Orcas island, Mt. Constitution is a big climb but has an incredible view of the whole sound, the Olympics, and the Cascades.