I own both a top notch racing Road bike(TCR0) and a top notch racing cross bike (Kona Major Jake). They are both very very fast on the road. At speeds above 25 mph on the flats the TCR begins to be more efficient. Below that speed, the cx bike fit with road or even hybrid tires (I use the Michelin Transworld Cities) are about equal with a slight edge going to the cx bike if the road is rough chip seal or something.
I absolutely love my CX bike and just about always chose it over the road bike (unless I know it is going to be a slug fest).
We did the MS 150 this weekend and spent two days well above 23 mph (we were usually running around 25 but often up to 28 on the flats). I was on my road bike and near my limit all the time but my buddy was on his cross bike with well worn Ritchey Speed Max Knobbies and was not only hanging but often driving the speed up.
As long as your rides on the road are not races, you will be just fine out there on a cx bike with the right tires (and even with knobbies
).
Moreover, my cx bike will climb incredible off road trails without a 2nd thought (even with slicks). It just isn't the best off road descender.
Get a cx bike if you have a desire for one. You wont be sorry if you do mixed riding. I commute about 30 miles/day on mine and LOVE IT.