Cheapskate alternative:
wiggle.com | dhb Aeron Super Light Gilet | Cycle Vests
I was going to get the Sportful hot pack but gave this one a shot this one a shot for 1/2 the price. This vest will get lotsa usage, very lightweight but keeps the wind off on those transitional days and packs down to nothing. Has mesh side vents too. Size down for race fit, runs a little loose.