Well I have an iPhone so I can tell you my experience. It works! To use it just as a phone, receiving calls and texts is no different than your regular plan. To make calls or send texts there is an international rate but it's so cheap I didn't even notice it when my bill came. Just call AT&T from your phone and they can tell you what it is. I called them on my way to the airport to make sure I didn't need to do anything special and I didn't. (When you turn your phone on in Jamaica, it will connect to the Digicel network.)
But to use it as anything other than a phone, it will cost you more money to roam while searching for a wireless network or the 3G network. So I turned all of these features off when I was there (location services, data push and wireless network). You can still play games though. Hope this helps! BTW, you will LOVE CTI!!
This link has the roaming rates for AT&T.
If you want use data I would strongly consider one of the Global Data add-ons. They range from 20MB ($25) - 200MB ($199).
The default data roaming charge below are very high.
INTERNATIONAL WIRELESS ROAMING PER MINUTE RATES
Calls Placed/Received While in Jamaica
AT&T Standard International Roaming $1.99
AT&T World Traveler $1.69
AT&T Canada $1.99
AT&T Mexico $1.99
Data Usage: All GPRS pay per use rates are $.0195/KB, except in Canada rate is $.015/KB
Text, Picture and Video Messages:
Send Text Message $0.50 per message sent
Send Picture and Video Messages $1.30 per message sent
Receive Text, Picture and Video Messages
Home pay-per-use or package rates apply to all text, picture or video messages received when roaming internationally.