Its a very low-key, very hilly gravel grinder in the driftless region. People ride it on anything and everything. Cx bikes, fat bikes, mountain bikes, single speeds (i wouldn't recommend that). I rode it last year on a 1x9 steel cx with 35mm tires.
I would not recommend a road bike with 25mm tires. It gets very bumpy and chunky in places. You will want to run lower pressures.
I am considering doing it this year on a 29er hardtail with a rigid fork.