Last week i purchased a cyclocross bike which is 20#, I also have a 13# road bike. While the CX has 35mm tires the road has 25mm. It's night and day, while the CX feels like a branch, the road feels like a twig. The CX is a great bike but the road just accelerates quicker, climbs easier and the handling is superb. The CX feels solid and it awesome on descents.
Each is a tool for a slightly different job... which is why i purchased the CX bike.