The Illinois Depart of Natural Resources has published a guide to the Route 66 Trail which was produced by the League of Illionois Bicyclists and the DNR. The guide is available from the DNR and on-line. The guide's "Main Route" deviates a lot from Route 66 onto nearby roads, requiring many turns and adding significantly to the riding distance. Generally, I found riding on Route 66 in sections the guide notes as "Advanced Shortcuts" or doesn't list at all to be reasonabe riding. Some of that was 4-lane or 2-lane roads with no shoulders, but traffic was generally light and drivers gave wide clearance. The guide lists services available in many of the towns along the route.
The "Mother Road" is essentially intact and generally makes for fine riding. Much of old Route 66 is now frontage roads to I-55; in other areas it about 1 to 3 miles from the interstate (either way, the interstate carries almost all the traffic). The pavement is generally good, with a few areas with rough surfaces. Off-the-route trails from the Chain of Rocks bridge to Staunton, into and through Blooming/Normal, and at a few other places provide pleasant traffic-free breaks. There are small towns at about 5 to 10 mile intervals, so services are readily available. The towns are all viable, a switch from the ghost towns and abandoned buildings along Route 66 in the western states. Pontiac has an interesting Route 66 Museum which is a worthwhile stop.
I combined the Katy Trail in Missouri and Illinois Route 66 into one tour. A nice benefit of the entire route (in my view) is the lack of hills the entire tour. Crossing bridges were the biggest climbs of the trip.