I've always felt that while lighter bikes really don't do anything for you [Within reason.....I mean, you don't want to ride a 35 lb. tank....], lighter bikes do seem like they just "feel" better than heavier bikes- but I question how much of that better feel is due to the relatively small weight difference, vs. the fact that heavier bikes often have more "relaxed" geometry, and slower handling? [Which is probably what accounts for their less-nimble feeling, as opposed to a few pounds.]

My first road bike weighed 24 lbs., I now have a 17 lb. bike. Very little difference in my performance, unless you count seconds- which I don't. I like the "feel" of the 17 lb. bike much more- but I think that's because it is an aggressive-geometry race bike vs. the heavier bike, which was a relaxed geo. Comparing the 17 lb. newish CF bike to my 19 lb. old AL bike, I actually prefer the latter...and notice no difference at all as far as the weight. If I were blindfolded, I wouldn't be able to tell which was the heavier one.

I've heard it said that 12 lbs. off the bike/ider = 1 MPH. It sounds about right to me.