[inquisitor]

They charge as many connections as sessions you've established. Theoretically a session could last days, but practically it will be interrupted at least once per day. If you move with your phone during a session and so lose the signal for a moment or if a bad handover occurs, the session will also be interrupted.
Another problem I fear is, that if you initiate a session through your computer, it'll be closed if you switch it off. Most mobile phones (like my Nokia N95) will keep the session alive even at idle, if it has been initiated from the handset itself. But I don't know if you can join a handset-established session without interruption from you computer.
Against this background the ICQSIM seems more attractive me, as they charge only a daily usage fee of € 0.39 instead of session fees.