<asp:TextBox runat="server" ID="txtComment" Width="650" />

<asp:Button runat="server" ID="btnSubmitComment" Text="Submit" />

$(document).ready(function () {
    $('input[id$=btnSubmitComment]').click(function () {
    var comment = $("#txtComment").val();
    alert(comment);
});

$(document).ready(function () {
    $('#<%=btnSubmitComment.ClientID%>').click(function () {
    var comment = $('#<%=txtComment.ClientID%>').val();
    alert(comment);
});