I rode the ACA Lewis and Clark route from Portland to Missoula a few years ago. I took the Oregon side as far as I could, then rode across the bridge at Biggs and continued east on 14 on the Washington side.

I spent quite a bit of time on I-84. Most of the time it was "fine" (I can't think of a more descriptive word.) The shoulder is very wide and relatively smooth. Hills are climbed with relatively easy grades. The biggest downside was the noise - it's a busy freeway with lots of semi trucks.

There was one place where I was supposed to get off the freeway and I didn't (due to some glitches with the gps I was following.) It's the famous section where cyclists go up and over and there are wheel grooves in the cement next to the steps. I missed it and ended up riding through a freeway tunnel with no shoulder. It was scary and loud but I made it through. I guess it wasn't any worse than some of the other scary places I've ridden on tours - the bridge across the Columbia into Astoria, the highway into Whitefish (where I missed another turnoff,) the tunnels on the Oregon coast - there was another scary spot just ahead of this one - the bridge across the Columbia from Biggs into Maryhill.

There were a couple other places where I had to circle to find the correct route on and off the freeway. ACA tries to get you off busy roads whenever it can, and sometimes it might be easier to just stay on the main road, rather than wandering around looking for the route (which often adds a few miles.) Don't get me wrong; usually I appreciate the bucolic roads that ACA chooses, and am pleased enough that I don't mind the few extra miles they might add.

Anyway, I recommend the ACA route maps, and I recommend using them to navigate while riding, rather than just relying on a gps.