This trail extends from South Beloit, IL to west of Harvard, IL. Main trail reference is on the Rails to Trails page. I recommend reading comments there as well if you want to travel the trail.
I rode the trail a couple weeks ago, from Caledonia headed out of Rockford, heading east to Harvard. After the hills and back roads to get there from Rockford, I was very happy to see the trail which is shaded, has stopping areas, and is a beautiful ride on a rough surface.
The trail surface, in some areas, is in very bad shape. I had no trouble riding it on my road bike, but just be aware that the cracks in line with the trail can seriously damage a wheel. That didn't stop a lot of people from riding the trail and if you don't mind dodging cracks, it is worth the trip at least from Caledonia eastward.
There is a parking lot at the eastern terminus of County Line Road. A lot of people seemed to be starting from there. I headed down County Line Rd then went East on E Main St which felt relatively safe with a good shoulder for the ride into Harvard. Happy riding!