2 Felt F1X's, Surly Steamroller Fixed / Free, 2007 IF Crown Jewel, 2007 IF Planet X Singlespeed
yes i have raced quite a few xc courses on my cross bike. the main drawback (from my perspective) is the tires, and the ability to bomb the descents. you have to have alot of finesse to pick your way through rocky descents, but you can make up serious time on the flats and hills (if your gearing is right). cx bikes are sturdy, but mtb bikes are generally more beefy. if you dump a cx bike in a rock garden you could easily break an expensive lever / fork.
I gave the CX bike a go at the local MTB races this season. I found it to be only ďOKĒ on the CX bike w/riser bars. Upside a lot more speed on the flats, style points for doing it on tall skinny wheels, one less bike in the garage.
Downsides are agreed with XCCX. Itís difficult sometimes to pick a good line at a race speed, and you might dodge a big rock only to hit the one medium sized one you didnít see at race speed. Thus you end up blowing a tire AND OR thrashing a rim. Iíve had both experiences. Iím not afraid of rocks on the cx bike, but Iím fearful of what they can do to the bike. Check out the course, if itís really rocky say 75% of course, save your wheels for another day.