Hi,
I have an 07 Cannondale cyclocross disc Optimo 2 and I love it. Discs on a road bike are awesome.
I use this bike for everything. Mine has a 105 rear, and I upgraded to 105 brifters. I also run a Brooks Swift saddle for a touch of retro. I run road tires almost all the time and ride group rides with the local bike club with no problem.
Note the Disc and Canti Cannondales are very different frames, and the CAAD9 Cyclocross frame has little to do with the Road CAAD 9. My Optimo frame looks more like my wifes CAAD8 road frame than the Canti frame with its unified upper seat stay. My seat stays are curved and really ride nice even though the whole bike is alu even the fork. The down tube is massive, this is a seriously stiff main triangle yet it rides great on road and off.
I am sure you will love your new cannondale.
Brooks