The following xaml code

            <telerik:RadComboBox Margin="5"
                                 ItemsSource="{Binding Path=AvailableServices}"
                                 IsEditable="True" 
                                 Text="{Binding Path=Service, Mode=TwoWay}"/>

            <ComboBox Margin="5"
                                 ItemsSource="{Binding Path=AvailableServices}"
                                 IsEditable="True" 
                                 Text="{Binding Path=Service, Mode=TwoWay}"/>

with this view model

public class ViewModel
    {
        public ObservableCollection<string> AvailableServices { get; set; } = new ObservableCollection<string> { "UPS Standard", "UPS Express Saver", "UPS Express 12:00", "UPS Express" };

        public string Service { get; set; } = "UPS Express";
    }

produces the attached output. 

The standard WPF ComboBox is set to "UPS Express" as expected while the RadComboBox shows "UPS Express Saver" which probably was incorrectly autocompleted. 

It looks like the `Fluent` themes may not properly theme the Non-Editable RadComboBox control.

Within the default style for RadComboBox within Telerik.Windows.Controls.Input.xaml, there exists a RadToggleButton with the name "PART_DropDownButton" which is part of the main control template. This toggle button uses the style "NonEditableComboToggleButtonStyle", which then sets the toggle button template to the "NonEditableComboToggleButtonControlTemplate" control template.

This control template has a Border named "ToggleButtonBackground" where to background is set to a template binding. However, the NonEditableComboToggleButtonStyle does not set background, and it therefore leads to a control that looks like the attached image.

This can be fixed by overriding the RadComboBox style, but it isn't simple to set the background on the ToggleButton, so you need to create custom templates and style to do it.

If you set IsEnabled = false, in the RadComboBox then (in the Office 2016 theme) the entire box is faded, including the text. A suggestion in the forum was to set IsHitTestVisible to false, as well as IsTabStop. This works, but it means that the dropdown button still looks enabled. It would be nice if IsReadOnly did what I wanted, but it doesn't. 
Therefore, either a way to set the Visibility/Opacity of the dropdown button. Or alternatively, modify the theme(s) to make the text more visible when the box is IsEnabled is set to false, in the same way as the standard ComboBox. 
I'm aware that I can modify the template myself, but it'd be nicer if it was built in. 

After making a selection in the control and changing the theme two times in a row an InvalidOperationException is thrown about the animation.

When the app is in full screen OOB mode the placement of the drop down portion of the ComboBox control is incorrect.

Workaround: use OnDropDownClosed event.

Specifically this bug was found in WPF, I'm not sure of its state in Silverlight.
The FontWeight is permanently some shade of Bold when using Metro.
This is demonstrated in the Telerik WPF demos in the ComboBox/Theming demo.

MouseDown and MouseMove are not handled by RadComboBox dropdown when in XBAP, so if the RadComboBox is in Window, it can be resized while the dropdown is opened.