That is a nice design. Using the saddle rails makes it look like it would be a lot stronger than other seatpost racks, but only rated to carry 13lbs. I guess you carry more with it unless you have super lightweight saddle/seatpost.

Another rack I forgot, the best ones I know of for bikes without rack mounts are made by Old Man Mountain and I was going to find a link for them after looking at the Arkel site, but I found them on the same website. http://www.arkel-od.com/us/coldsprings-rear.html

I don't know how Old Man Mountain and Arkel are related. I don't think Arkel makes OMM racks, but they sell them.

Tubus racks are very good too, so hopefully the Cannondale will have the rack mounts. Cannondale made panniers before they made bicycles and I think rack mounts are still pretty standard on most of their bikes.