I will be riding my Cervelo S5 for the fifth time across Iowa. The other six Ragbrai I have done have been on road bikes as well. I have read a lot of suggestions on what to ride however I believe it comes down to ride what you want to ride. If you would feel most comfortable on a hybrid ride it, if you like the preformance of a road bike ride it, if you want the challenge of a unicycle and can, ride it. The great thing about Ragbrai is that there is not a correct way to do it. Be happy and ride it however you want and ride whatever want. Just have fun!