My 3G was jailbroken for a theme/SB settings/cycorder, I think that was about it. Had the iNav theme to it kinda made the phone real sluggish. But yeahhhhh.
When they get a jailbreak made i would jailbreak it! I do not think it will hurt the 3GS (near as much) because it has more CPU power (600mhz vs 412mhz) and more ram (128MB vs 256MB ).