Have not long bought a Giant Defy Advanced4 running Shimano 105 with FSA Gossamer chainset and Mavic Aksium Race wheels and love it. Did a 55mile ride yesterday with lots of climbing (Chester, up and around the Clwyds) and feel totally fresh today. IMO, its worth the extra for carbon as it totally eliminates that road buzz that gives you numb bum/hands.
Its fast, spins nicely at 23-25mph on the flat, descends really well is utterly stable when pushing 40mph downhill and is proper all day comfy.
Oh it cost 1500 pounds and weighs 19.85lbs with pump, computer, bottle cage and pedals on a large frame