its not like " A LOT" lighter but tis a tad bit lighted than the stock one. I'm thinking about 3.73's in the near future. Did you notice a big difference? i have the stock gearing right now which i think is 2.68.
It's only a few pounds, but what you have to consider is that rotating mass weights more than static mass, so when it's spinning 5k rpm, it's a WHOLE lot less than a stocker.
Combine a new, good quality harmonic dampener with an aluminum ds and your car will be much smoother. On a bone stock car, the shaft wont make huge improvements but they're enough that you should notice, and the dampener is great to smooth out your motor and will actually add a few hp compared to an older stock unit.