If so how?
Did you roll your own with jQuery or use the Microsoft AJAX toolkit?
Did you create a webservice or call an action?
EDIT : Please note that this question was asked before Microsoft announced that they were going to bundle jQuery in with VS/ASP.NET MVC. I think there is less of a discussion around this topic now and the answer would now almost always be 'use jQuery'. IainMH 11th Feb 2009