iOS 4 currently can't really be Jailbroken smoothly on iPhone 3GS, but check out the How-To guide in upgrades to jailbreak iPhone 3G on iOS 4, 3GS to come very soon.
You can also unlock the baseband on iPhone 3G if you do update, see the How-To in upgrades.
Leave feedback on the OS, smoothness, what you like/don't like etc..
As you may know iOS 4 brings multitasking without killing your battery, as well as folders, the ability to change your wallpaper without jailbreaking and many more OS-wide features.
check
this on the Apple site for features ("over 100 new features)