Im an s3 user, just reading this thread to see if worth the change to s4. Just wanted to post coz many are asking about maxis.
My mom is using maxis, they will throttle down after the data limit. BUT u must beware of the hidden charges, hard copy bill, call waiting, call barring, mms spam, and many others. They charge all this. After u register, go to ur online account and cancel all this.
Another thing, my mom once got sms spam, she disregard this, but every time she dont know y her bill always reach limit after 2 weeks. Its bcoz these sms charge 50sen,total was rm38 if im not mistaken.
Always check ur bill.

Yes.

Im actually not that sure about call waiting, but i do know all service is enabled by default when u register n they do charge many of them. So just to be safe better check ur online account and disable which u do not use. They do have state which service is charged and how much.