Well, actually you might not notice it.
The difference in 0.5kgs is, IMO, only noticeable on longer (more than 25 mile) rides over lumpy terrain where you're likely to feel less tired which I guess, is a result of accelerating easier up lumps/hills.
That is pretty much what I've found. The wheels on my carbon bike are about 400g lighter than the other and up slight/moderate hills it feels easier, or I climb quicker. If it gets really steep, I'm still struggling, but over a decent distance I do feel less tired on the lighter wheels.
Like all things there is going to be a point of diminishing returns. The Defy wheelset weighs in at over 2kg (yeah, they're pretty heavy) and the others are about 1700g, so that's a significant difference. If I then went to a 1500g set, I would probably not notice such a massive difference.
