well with a jailbreak via installouse u can get apps for free and winterboard has fulll themes instead of one picture in the background plus u can change you fonts and download music and videos from your ipod and sync them to you ipod via wifi
If you have a 3GS, then no, there are no advantages unless you want apps that don't have to go through the App Store approval process. If you have a 3G, then jailbreaking is still an attractive option as the 3G phones do not get wallpapers, multitasking, or bluetooth keyboard support.