I have two Cannondale cyclocross bikes, one aluminum (CAADX) and one carbon fiber (SuperX). They have similar geometry and the total bike weights are pretty close.
Components aside, the SuperX feels much faster and lighter on the road, accelerates quicker and climbs easier. It's also significantly smoother over rough stuff like gravel, dirt trails, etc.
Before I bought the SuperX I wasn't sold on the need for a carbon frame. I had done a few test rides on carbon bikes and they never felt like a huge step up from an aluminum frame. I've also always liked the durability of aluminum, especially for a CX bike that will get thrashed around a race course and crashed a bunch of times. I would've happily bought another aluminum frame CX bike, but no one seems to put anything nicer than 105 on aluminum frames anymore.
After owning the SuperX for a little while and riding it on a variety of terrain, with a few different sets of tires, I'm totally sold on the improved ride quality of carbon. The only reason I'd buy another aluminum bike is to either save money, or because of durability concerns. I'm a little concerned about what kind of condition my SuperX will be in after a few seasons of CX racing.