I have a jailbroken 2.2 3G AT&T Legit plan. - nothing wrong with mine
No surprise charges unless you use PdaNet unresponsibly and ATT catches you
No phone disconnections
Haven't lost service unless i hit those dead zones around where I live.
No disadvantages, only advantages
To Jailbreak - Use quickpwn. Download quickpwn from the dev team torrent and make sure you uncheck Activate Phone since you are with ATT. After that, you should have Cydia and Installer shown on your springboard.
any other questions just go to here if you have a mac or here if you are running windows. Good luck