A few thousand miles back I replaced my Araya wheel set with a Sun wheel set, and the new hoops were much lighter. It was fun accellerating from stops and on climbs, but I noticed a lack of stablity at times, especially on descents over rough pavement. (Being a Rivendel, my bike is very stable. But I really dislike any twitchiness, especially on the front end.)
The other day I bought a set of thorn resistant tubes, just to see what kind of flat resistance they offered. Of course, they were much heavier that my standard tubes, and I really felt the weight increase as I rode. However, they were not so bad on long flat sections and descents, where the added stabilty of the weight overcame the sluggish feeling. After a few long rides with the heavy tubes, I'm yearing for something, weight-wise, in between my light tubes and heavy tubes.
Other than for racing or fast group rides, maybe the the ideal wheel weight isn't "the lighter the better," but something in between???