Workaround: 

Use the new PivotGridSpreadExport class: http://www.telerik.com/help/winforms/pivotgrid-export-srpead-export.html

To reproduce:
Set the decimal separator to comma(,) in your regional settings and export the RadPivotGrid using PivotExportToExcellML. Open the exported file and you will see that the font size is very big.
Workaround:
Subscribe to the PivotExcelCellFormatting event and set the font size to an integer value:
void excelExporter_PivotExcelCellFormatting(object sender, ExcelPivotCellExportingEventArgs e)
{
    e.Cell.Font = new Font(e.Cell.Font.Name, (int)e.Cell.Font.Size, e.Cell.Font.Style);
}

To reproduce:
Resize some of the columns in PivotGrid and then export it with PivotExporttoExcelML. All columns in exported file have the same width.
Workaround:
Use PivotGridSpreadExport.

To reproduce:
In RadPivotGrid set some cell values to start with "=" or "-". Export it using PivotGridSpreadExport. The exporter tries to convert the value to formula and if this is not successful an exception is thrown.

Workaround:
private void radButtonNewPivotToExcel_Click(object sender, EventArgs e)
{
    SaveFileDialog saveFileDialog = new SaveFileDialog();
    saveFileDialog.Filter = "Excel files (*.xlsx)|*.xlsx";
    if (saveFileDialog.ShowDialog() == DialogResult.OK)
    {
        SpreadExportRenderer renderer = new SpreadExportRenderer();
        MyPivotSpreadExport exporter = new MyPivotSpreadExport(this.radPivotGrid1, renderer);
        exporter.ExportFormat = SpreadExportFormat.Xlsx;
        exporter.RunExportAsync(saveFileDialog.FileName + ".xlsx", renderer);
    }
}

public class MyPivotSpreadExport : PivotGridSpreadExport
{
    private ISpreadExportRenderer spreadExportRenderer;

    public MyPivotSpreadExport(RadPivotGrid pivotGrid, ISpreadExportRenderer renderer)
        : base(pivotGrid)
    {
        this.spreadExportRenderer = renderer;
    }

    protected override void AddRowToWorksheet(PivotGridExportRowElement rowElement)
    {
        foreach (PivotGridSpreadExportCellElement exportCell in rowElement.Cells)
        {
            this.spreadExportRenderer.CreateCellSelection(rowElement.Index, exportCell.ColumnIndex);

            Color borderColor = this.ShowGridLines ? exportCell.BorderColor : Color.Transparent;
            this.spreadExportRenderer.CreateCellStyleInfo(exportCell.BackColor, exportCell.ForeColor, exportCell.Font.FontFamily, exportCell.Font.Size,
                exportCell.Font.Bold, exportCell.Font.Italic, exportCell.Font.Underline, exportCell.TextAlignment, exportCell.TextWrap, BorderBoxStyle.SingleBorder,
                borderColor, Color.Empty, Color.Empty, Color.Empty, Color.Empty);

            if (exportCell.DataType == DataType.String || exportCell.DataType == DataType.Other)
            {
                this.spreadExportRenderer.SetCellSelectionFormat("@");
            }
            this.spreadExportRenderer.SetCellSelectionValue(exportCell.DataType, exportCell.Value ?? exportCell.Text);
        }
    }
}

Add functionality which will allow the end users to edit the aggregated data. The new values should be passed back to the business logic of the application which will decide how to divide the new value across underlying records.

Wokaround:

Private Sub GroupDescriptorElementCreating(sender As Object, e As Telerik.WinControls.UI.GroupDescriptorElementCreatingEventArgs)
    RemoveHandler e.GroupDescriptorElement.FilterPopup.TextBoxMenuItem.TextBox.TextChanged, AddressOf TextBoxMenuItemTextChanged
    AddHandler e.GroupDescriptorElement.FilterPopup.TextBoxMenuItem.TextBox.TextChanged, AddressOf TextBoxMenuItemTextChanged 
End Sub

Private Sub TextBoxMenuItemTextChanged(sender As Object, e As EventArgs)
    Dim tb As RadTextBox = TryCast(sender, RadTextBox)
    Dim popup As PivotGroupFilterPopup = TryCast(tb.Parent, PivotGroupFilterPopup)
    popup.TreeViewMenuItem.TreeElement.TreeView.EndUpdate()
End Sub

Workaround:
public Form1()
{
    InitializeComponent();
    this.radPivotGrid1.GroupDescriptorElementCreating += radPivotGrid1_GroupDescriptorElementCreating;
}

private void radPivotGrid1_GroupDescriptorElementCreating(object sender, Telerik.WinControls.UI.GroupDescriptorElementCreatingEventArgs e)
{
    e.GroupDescriptorElement.FilterPopup.PopupOpened -= FilterPopup_PopupOpened;
    e.GroupDescriptorElement.FilterPopup.PopupOpened += FilterPopup_PopupOpened;
}

private void FilterPopup_PopupOpened(object sender, EventArgs args)
{
    PivotGroupFilterPopup filterPopup = (PivotGroupFilterPopup)sender;
    RadTreeView tree = filterPopup.TreeViewMenuItem.TreeElement.TreeView;

    ((RadScrollBarElement)tree.TreeViewElement.Children[2]).Value = 10;
    ((RadScrollBarElement)tree.TreeViewElement.Children[2]).Value = 0;
}

Workaround: 

private void radPivotGrid1_GroupDescriptorElementCreating(object sender, Telerik.WinControls.UI.GroupDescriptorElementCreatingEventArgs e)
{
    e.GroupDescriptorElement.FilterPopup.PopupOpened -= FilterPopup_PopupOpened;
    e.GroupDescriptorElement.FilterPopup.PopupOpened += FilterPopup_PopupOpened;
}

private void FilterPopup_PopupOpened(object sender, EventArgs args)
{
    PivotGroupFilterPopup filterPopup = (PivotGroupFilterPopup)sender;
    RadTreeView tree = filterPopup.TreeViewMenuItem.TreeElement.TreeView;
    int height = this.CalculateHeight(tree);
    if (height > tree.Height)
    {
        ((RadScrollBarElement)tree.TreeViewElement.Children[2]).Value = 0;
        ((RadScrollBarElement)tree.TreeViewElement.Children[2]).Maximum = height;
        ((RadScrollBarElement)tree.TreeViewElement.Children[2]).Visibility = Telerik.WinControls.ElementVisibility.Visible;
    }
}

private int CalculateHeight(RadTreeView tree)
{
    int height = 0;
    for (int i = 0; i < tree.Nodes[0].Nodes.Count + 1; i++)
    {
        height += tree.ItemHeight;
    }

    return height;
}

When a filter dialog is shown (Report Filter or Label Filter) for OLAP Hierarchy, it contains the items only for the selected level. Instead items from all levels could be visible, so you won't have to open additional dialogs to filter them (show them like a tree).

Workaround: 
PivotExportToExcelML.ExportVisualSettings = false;

To reproduce:
- Add calculated field
- Try to set its number format, by clicking the Number Format option in the context menu in the field list (see image)

Workaround:
 Private Sub RadPivotGrid1_CellFormatting(sender As Object, e As UI.PivotCellEventArgs) Handles RadPivotGrid1.CellFormatting
        If e.CellElement.Column.Name = "Commission" Then
            e.CellElement.Text = String.Format(System.Threading.Thread.CurrentThread.CurrentUICulture, "{0:C}", e.CellElement.Value)
        End If
    End Sub

To reproduce:

1. Add a RadPivotGrid and  change its theme to TelerikMetro. This theme uses Segoe UI font for its cells.
2. Export the RadPivotGrid.
3. You will notice that the font is applied to some of the cells, but for others, e.g. Grand Total the default font is set to Microsoft Sans Serif

Second scenario: use Demo application PivotGrid >> Export to Excel example. Use ControlDefault theme.
1. Ctrl + H to show the spy.  Check font for row  "Jun-1 >> Sum of Net", column "Printer stand >> 1 Free with 10".  The font is Segoe UI.
2. Run the export and check the same cell in Excel. Its font is Microsoft Sans Serif.

Workaround: use the PivotExcelCellFormatting event to specify the font.

Argument exception when a specific layout is loaded:
"An item with the same key has already been added."

One should be able to move the selected cell and scroll with the arrow keys. The attached project features a solution for moving the currently selected cell using the arrow keys.

Workaround:

        void exporter_PivotExcelCellFormatting(object sender, ExcelPivotCellExportingEventArgs e)
        {
            double d;
            if (double.TryParse(e.Cell.Text, out d))
            {
                e.DataType = "double";
            }
        }

To reproduce: use the following code snippet. It is reproducible in target framework less than .NET Framework 4.0.

Sub New()
    InitializeComponent()
    Me.RadPivotGrid1.RowGroupDescriptions.Add(New DoubleGroupDescription() With { _
        .PropertyName = "EmployeeID", _
        .GroupComparer = New GroupNameComparer() _
    })
    UpdatePivotGrid()
End Sub

Private Sub UpdatePivotGrid()
    Dim ds As DataSet = New DataSet
    Dim conn As New SqlConnection("Data Source=.;Initial Catalog=Northwind;Integrated Security=True")
    Dim cmd As New SqlCommand("SELECT * FROM Orders", conn)
    conn.Open()

    Dim da As SqlDataAdapter = New SqlDataAdapter(cmd)
    da.Fill(ds)
    ds.Dispose()

    Me.RadPivotGrid1.DataSource = ds.Tables(0)
    Me.RadPivotGrid1.Refresh()
End Sub

Please refer to the attached gif file.

Workaround : use .NET Framework 4.0.  

Workaround:

  private void radButton1_Click(object sender, EventArgs e)
        {
            radPivotGrid1.RowGroupsExpandBehavior = new GroupsExpandBehavior() { Expanded = true };
            radPivotGrid1.ColumnGroupsExpandBehavior = new GroupsExpandBehavior() { Expanded = true };
            radPivotGrid1.DataProvider.Refresh();

            ExportTo.ExportToExcel(this);

            radPivotGrid1.RowGroupsExpandBehavior = new GroupsExpandBehavior() { Expanded = false };
            radPivotGrid1.ColumnGroupsExpandBehavior = new GroupsExpandBehavior() { Expanded = false };
            radPivotGrid1.DataProvider.Refresh();
        }