I built up a 5.9kg fixed bike. Stupid, too light. I can't see how this would be fun to ride on a decent or cross wind. Still impressive.
My road bike is around 6.1kg including Zipp 303 aero wheels. With my Mavic wheels it dips below 6kg. I've never found it to be a problem in cross-winds or descents - and I live on the coast, winds can be fairly strong here. However, I can imagine sub-4kg being a challenge.
It is great as a climbing bike and generally fairly quick (more KOM's on this than my previous bike), very responsive albeit not as outright fast overall on the flats as an aero bike. Where I would criticise it most though is for sprinting; under full gas the rear wheel does move around a bit scrabbling for traction. It's also not a bike for taller, heavier riders, I'm short, 65kg and I think this bikes recommended limit is 80kg including bike + accessories!
I'm definitely interested in lighter bike tech but would prefer to see a better lightweight/aero combination.