Just to open up the CX bike debate a little more...
They're perfect for one thing. CX races! Well, CX race bikes are perfect for that (the Croix De Fer for instance is more of a grand tourer that can do a bit of everything), but for everything else they're a compromise. But sometimes it's just how much (or how little) of a compromise they are that matters.
I borrowed a Whyte Saxon Cross for a few weeks recently, and to be fair, it wasn't much slower than my road bike on the road even with the 33c knobblies on (with 75psi in mind). OK it was slower, but not even 10% slower. Put a set of 25C slicks on and I'd bet it was only 2-3% slower than my full carbon/ultegra road bike. But then you can take it offroad too. OK, so I'd not want to descend a World Cup downhill on a CX bike, or even a rocky descent at Afan, but for most people's local trails, you will find a CX bike is as quick, if not quicker than an MTB pretty much everywhere! I was shocked just how good it was offroad, and will definitely be buying one in the near future!
In fact, I'm even debating getting rid of my road bike and just buying a CX bike like the Whyte Saxon Cross, and then a second set of lighter wheels to keep a set of 25c tyres and a closer ratio cassette on for road ride duties.