jailbreakme.com doesn't work with the 4S.
here you go homie:
http://www.redmondpie.com/jailbreak-iphone-4s-ipad-2-untethered-on-ios-5.0.1-using-absinthe/
and being able to customize the font is minor compared to all the shit you can do while jailbroken:
free tethering/wifi hotspot
free apps/games
themes that customize almost every aspect of the phone's interface (besides the font)
various tweaks that add functionality to the phone
etc.
try googling "top cydia apps" or "top cydia tweaks" or "best winterboard themes" to get an idea.
good luck