I am currently using a Verizon 6600 and I am VERY happy with the feature (of the Windows Mobile OS and thelatest version of Exchange server) that allows Active Sync to ocurr any time there is a change on the Exchange server by apparently sending a text message to the device and this causes the device to initiate a sync which happens very rapidly. This allows for almost instantaneous receipt of emails and even changes in your calender initated from the office (and sync of all devices almost immediately - my work PC, my home PC, my laptop and my PDA/phone).

I have to keep a GSM account for when I go overseas so I am considering getting a (unolocked) Treo 650 to use on that Cingular account. What experince do you guys have with different systems that allow for push email and or calender / contact sync with an Exchange server? I know that apparently the native Versamail client is supposed to do something along these lines but could really get feel for the functionality by reading the Palmone website.