Here is a whole thread about the efficiency of lighter weight wheels vs the inertia of a heavier wheel set:

http://www.bikeforums.net/showthread...1#post13353109

I bought a set of 1000 gram wheels that dropped the weight of my bike by 2 pounds, the biggest difference I notice is on take off, and really steep climbs, not much difference in overall times. Having a 13 pound bike is really good for bragging rights, training your ass off is better for performance.