The FSAA is more useful at lower resolutions, because you don't suffer a huge frame rate drop, and jagged lines are much more of a problem at lower rez than higher rez. There are quite a few differences. The overall image is much smoother. Jagginess is almost completely indiscernible. On the other hand, lettering becomes fuzzier, making it harder to read.
Some textures also lose their flair in certain instances.
Click each image and judge for yourself. Sometimes it's more useful in certain games and certain hardware. The 3dfx MacTools control panel indicates that you can get better image quality with VSync turned on. Sometimes I noticed graphical imperfections such as static lines and jerkiness with VSync off. Open GL often performs better with VSync on.