Weight does make a difference. How much weight it takes to make a significant difference depends on what the person is trying to do with the bike. A top flight competitor will need to pay attention to the grams. A fit fast rider the pounds. An intermittent rider probably doesn't need to worry about weight at all. Mind weight is total weight. Doesn't do a whole lot of good to spend lots of money to make a bike lighter when a fitness program will both save weight and increase strength.

Don't forget gearing. A heavy mountain bike like my Giant Revel 29er is properly geared for slow speeds up hills. Pedaling harder doesn't get you much more speed. But, it sure does climb, slowly climb. On the other hand my lighter and higher geared LeMond probably has no limit short of the speed of sound if the engine is strong enough.

Two days this week I've ridden the 20# heavier Giant on trails I've often ridden in the LeMond. It gets pretty tough going up hills but there is no fear I won't get there.

In short, my experience is that weight is a definite factor. That is Total Weight including rider and all the rest that is carried. But, so are gearing and tires. Different wheels I think only matter to the superior athlete. I suspect there aren't as many of them as posts might indicate.