Event after an Ajax call is completed

Hi there,

Maybe this is a very noobish question but I'm sorry.. I gotta present it or I'll go crazy!

$(document).ready(function() {

$("#USERS").dataTable({
"bJQueryUI": true,
"sPaginationType": "full_numbers",
"iDisplayLength": 5,
"bProcessing": true,
"bServerSide": true,
"sAjaxSource": gurl_tbl_users,
"oLanguage": {
"sSearch": "Search Ticket:"
}
});
}

First off all, this implementation makes the datatable access the ajaxcall URL, retrieving and showing quite nicelly my data.

My question is, how do I register an event or funcion (or listen to one already implemented) so I can, by instance alert() the number of rows retrieved from the server?

I saw this one:
$(document).ready( function() {
$('#USERS').dataTable( {
"fnDrawCallback": function( oSettings ) {
alert( 'DataTables has redrawn the table' );
}
} );
} );

but I think there is some initialization issues about it... an alert() pops up...

Thanks in advance for any help given!