I ride Interstate 5 from Vancouver, WA to Olympia, WA quite a bit. I like the wide shoulders most of the way, the direct nature of the route and the mellow hills. Already mentioned dislikes are the noise and the dirty shoulders.
Silver lining of the noise/traffic is that it makes me ride faster for some reason, adrenaline rush, the wind from vehicles pushing me down the road perhaps, I sometimes pray for more double trailers to come by and help pull me along.
Mr. Tuffies in the tires have kept me flat free in the debris field that is the e-lane for about 600 miles, knock on wood.
The earlier on a weekend morning you can start the better. A 4am-10am Sunday morning century wouldn't be too bad traffic wise. I always start late, big ol breakfast before hitting the road at 7 or so, then take pretty long lazy breaks at 35 and 75 miles, so only the first couple hours are light traffic for me.