Carbon fibre frames are generally slightly lighter than aluminium frames. But that is not the only reason it gets used a lot. I ride road bikes, and there Carbon fibre see's a lot of use, it can actually make for a far more comfortable ride, all but the cheapest of road bikes these days have carbon forks, the better models normally have carbon fibre seat posts as well, carbon fibre rear triangles are becoming popular, not for the sake of saving a few grammes but to make a more comfortable ride.
Carbon Fibre IS a great material for road bikes, its not just for racers looking at saving every gramme, its great for commuters and lesiure riders as well. It provides a lot of the comfort of a steel frame but with less weight than alu.
One of carbon fibres biggest draw backs is that when it goes it tends to go suddenly and completely it cracks rather than deforms, there are some quality issues with badly built carbon frames leading to early failures, but a decent quality carbon fibre frame is no less durable than a decent quality alu frame.