From central OH westwards, US Route 40 presents little in the way of scenic splendor. If you enjoy looking at mostly-flat rural farm country, and an occasional remnant of the golden age of the National Road, you might find it enjoyable. I'll provide a brief overview of the portions in OH and IN on which I've traveled.
From Terre Haute, IN to Brazil, IN the road is four-lane all the way (except in the cities themselves), with moderate to heavy traffic depending upon time of day. Note that for about a third of this distance (at the Brazil end), US 40 is following a bypass rather than the historic National Road.
In between Richmond, IN and Springfield, OH US Route 40 is a very straight, mostly-level, two-lane road with light to moderate traffic. There are two rather roundabout exceptions to the straightness, though, where flood control zones built to protect Dayton caused the road to be rerouted atop the dams several miles to the south, rather than via the historic route.
Between Springfield, OH and Columbus, OH it is a four-lane highway, divided in many places. Mostly straight and level, except for the climb out of the valley in which Springfield lies. Traffic is usually very light because I-70 runs parallel to it, never more than 2.5 miles away. This portion is almost entirely via the historic route, as the road was expanded onto adjacent farmland or the abandoned right-of-way of an electric interurban railroad, the Cincinnati & Lake Erie Railroad. Around the tiny villages of South Vienna and Summerford, US 40 diverts from the historic National Road to follow the route the railroad took. When US 40 was upgraded to a highway, in couple of places the eastbound lanes follow the railroad right-of-way while the westbound lanes continue along the historic National Road; this allowed construction of a four-lane highway without destroying houses along the south side of the road.
Expect traffic in Terre Haute, Brazil, Richmond, Springfield, and Columbus.