I have looked around for a video but haven't found one. And honestly you won't really experience a speed difference unless you are playing graphic intensive games.
I'll list my reasons why I chose to root
Customization (there are tons of themes for the Nexus S and for cm7 theme chooser.
Battery life (my batter life basically doubled.)
Voodoo sound control (better than any ipod or mp3 player in terms of sound quality.)
More features like being able to take screenshots, and use apps that require root.
Flashing and experiencing whatever you want there are a ton of roms and kernels that offer different features. For instance if you wanted to have sense, or TouchWiz there are roms for it. But they aren't exactly stable just yet.
There are plenty more reasons to root i just can't think of them at the moment.
