I've used it in Mexico multiple times.
The phone will connect to the 3G networks, yet the speed will be capped to 2G (128kbps) speeds. It works pretty good for basic/background stuff. You can use your intant messenger, get emails, turn by turn navigation (will take a minute to load and search, but once it's loaded it will work) and will even stream music (just have patience on the initial buffering)
Overall, I think it works decent enough. If you need to send attachments and stream multiple things and really "use" your phone and not just let it sync and keep everything updated, then you can purchase the data packages which honestly are pretty decently priced.