Originally Posted by Campag4life

Actually you counterdict yourself with your last comment. Ever ride a S5? Its STIFF. Venge? Slighty more forgiving but still STIFF. Why are there so many Venges on ebay? Because they ride like a cattle truck and most don't want to live with them day in an day out for 6W save over speeds of 25mph where most don't ride that don't ride bikes for living. There are tradeoffs with aero bikes as Robert pointed out. Drag really doesn't start to matter much above 20mph and its most dominated by riding position and not frame shape. As Roberts stated there is a reason why ALL frames aren't made with aero intent. Same applies to road wheels for example...Deep V wheels are radially STIFF. So you can spend a whole lot more for an uber stiff bike will questionable return...and most of that return will be a bone jarring ride.
PS: I would say 95% on this forum don't know why an aero frame is vertically less compliant or weighs more than a comparable non aero racing frame e.g. Venge versus Tarmac.