protected void Page_LoadComplete(object sender, EventArgs e)
{
    // If the Button is disabled
    if (!RadButton1.Enabled)
    {
        // bet the Button's client state
        string buttonClientState = Request.Form[RadButton1.ClientStateFieldID];

        // if there is no client state exit the function
        if (string.IsNullOrEmpty(buttonClientState)) return;

        // create a JS Serializer instance
        AdvancedJavaScriptSerializer JsSerializer = new AdvancedJavaScriptSerializer();

        // serialize the Button's state
        ButtonState btnState = JsSerializer.Deserialize<ButtonState>(buttonClientState);

        // ensure the Button's Enabled property reflects the Client-State value
        RadButton1.Enabled = btnState.enabled;
    }
}

// Create a Model for the Button's Client State
class ButtonState
{
    // the enabled property is more than enough
    public bool enabled { get; set; }

    //If you'd like to serialize more properties, you can do so by declaring them additionally.
    //public string text { get; set; }
    //public string value { get; set; }
    //public bool @checked { get; set; }
    //public string navigateUrl { get; set; }
    //public string commandName { get; set; }
    //public string commandArgument { get; set; }
    //public bool autoPostBack { get; set; }
    //public int selectedToggleStateIndex { get; set; }
    //public string validationGroup { get; set; }
    //public bool readOnly { get; set; }
    //public bool primary { get; set; }
}