Good answer!
Have a play on here:
http://analyticcycling.com/
Rider your weight, shaves 250g off front and rear wheel (each) rides up 5k long hill at 10% gradient (hefty hill!) and, all other things being equal, you'll be around 10 seconds or 30m quicker/faster.
We'll have different views as to whether that's significant or not.
Some things are not quantifiable in speed or time. Said lighter wheels may be more responsive, stiffer, spin longer on free wheel, have better bearings, ride smoother, look better, etc.
The difference between the Giant wheels on my Defy 1 and the Zonda's was huge. Very difficult to pinpoint exactly but felt so much smoother and faster. Strava proved it to be true, with 43 PR's on first ride on my regular route with no wind.
The bike was a lot more enjoyable to ride, that's my gain.
Since n+1 arrived, the Zonda's are now on that and Fulcrum 7's on the Defy. Defy doesn't feel quite so nice to ride now, but that's harder to quantify as n+1 has tainted it obviously!