About Unsprung and Sprung Mass:

People seem to be confused about sprung and unsprung mass/weight (force/load is the correct term we are looking at). Simply put without going into gyration effect, sprung force/load is just everything (for example the rider) sitting on top of the suspensions/springs (so the mass moves as suspension/spring travels, that's why it's called "sprung" mass) which is only affected by gravity which is basically constant as long as you are on Earth. Unsprung force/load is basically everything connected to the bottom of the suspension (these masses are between the road surface and suspension, and they don't move freely, instead they follow the road surface, that's why they are called unsprung mass) which has the same force/load due to gravity of Earth plus additional force/load when moving at speed due to centrifugal/spinning forces, given the mass concentration and geometry of the object is the same (center mass and moment arm for those of you knew Physics). To further simplify, suspensions only see the total vertical load coming from tires (if you push the tires up and down the suspension will move as well, but if you push the tire horizontally the suspension will not move at all). You can also look at load effect on suspension from chassis, but when pushing the chassis horizontally, suspensions will also move when you apply brake (we won't talk about this weight transfer using brake for simplicity). Sprung/dead mass will only give the suspensions a constant load (Force=Mass x Gravity) which will make it easy for the suspensions to deal with. With this said, adding sprung/dead mass to the bike is not that bad (an excuse for us to not lose weight!). Unsprung mass/weight on the other hand will give the suspension a constant changing load that is greater than just gravitational force/load except when not moving and during perfect cruising (the equation is complicated, but can be look at in form of Force=Force due to Gravity + Force due to Rotational Speed + Force due to Rotational Acceleration) which is hard for the tires (tractions) and suspensions (ability to keep tractions without upsetting the bike) to deal with (think about you dealing with someone that keeps changing his/her mind). The greater the acceleration/speed, the greater the force the unsprung mass generates. As you can tell now, unsprung mass will add additional weight/load to the bike at speed, where as the sprung mass will not add addition weight/load to the bike at speed. How big is the effect of saving 1 kg unsprung mass you asked? For most people, they will probably never feel any noticeable difference if you don't tell them about the weight saving of a lighter part first. But for racers and riders who take every turn hard (had to do with weight/load and gyration effect besides the fact that suspensions work better with less unsprung mass) and trying to accelerate faster (mostly weight/load effect), the lap time will justify/speak for itself (an overall lighter bike will be out run by an overall heavier bike but with 1 kg less unsprung mass on most race track). For those that must know by the numbers, an average sized wheel/tire (unsprung mass) experienced about "350" times the gravitational force (remember people such as pilots talking about G force?) as the same weighted sprung mass at 100mph/160kph (yes, the ratio is roughly around 350 to 1 if my calculation is corrected, that's why all racing teams are trying to reduce unsprung mass with carbon fiber parts). To conclude, aftermarket lighter wheels are very expensive if you are just buying for cosmetic reason. It's not too bad if you are into hard-core racing for the extra tenth of a second a lap you are looking for.