ASP.Net MVC3 JSON to Model

That's already handled for you by the framework.

So you define models:

public class MyViewModel
{
    public int Id { get; set; }
    public string Name { get; set; }
    public Complex Complex { get; set; }
    public IEnumerable<Foo> Foos { get; set; }
}

public class Complex
{
    public int Id { get; set; }
}

public class Foo
{
    public string Bar { get; set; }
}

then a controller action taking this model:

[HttpPost]
public ActionResult SomeAction(MyViewModel model)
{
    ...
}

and finally you hammer this controller action with a JSON request matching the structure of your view model:

$.ajax({
    url: '@Url.Action("SomeAction")',
    type: 'POST',
    contentType: 'application/json',
    data: JSON.stringify({
        id: 1,
        name: 'john smith of course, why asking?',
        complex: {
            id: 3
        },
        foos: [
            { bar: 'the bar' },
            { bar: 'the baz' },
        ]
    }),
    success: function(result) {
        alert('hooray');
    }
});