Old thread resurrected, so I thought i'd add my experience.
I have CAAD10s (aluminium) and a Focus Mares CX (carbon) and run the Mares with either 35mm Conti CX Speed tyres, or 23mm Conti GP4000s. Notwithstanding the less-dynamic nature of the handling of the Mares, when it is shod with the CX tyres it is a tad harder to push up to speed and does require you to expend slightly more energy to ride at your usual road bike speeds, but the difference isn't a whole lot. With the road tyres, it is easier to roll on.
I've ridden the Mares solo and in bunches (CX & Road tyres) and manage to keep the pace without too much drama, but wouldn't use it on our local fast bunch rides, as I reckon i'd suffer too much!
I do some commuting on the Mares and figure that if I am working a little harder, it is good for my fitness.
CX bikes open up a whole lot of variety and have the potential to be the one bike type that will do most - to a fairly high degree.
cheers