I jailbroke my Apple 3G phone and never once had a problem. At the time, AT&T had a limitation on downloading files that were too large via Wi-Fi only. Jailbreaking the phone allowed me to bypass that hurdle and download on 3G network. It also allowed me to play Pandora through my car radio. All in all, a very positive and rewarding experience.
Today, I own a 4G but as yet, have not rooted the phone. Plan to do so soon.
EC

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic