SPRadGrid supports the native SharePoint field types, however, the Q3 release version is the first to sport the possibility of binding the grid to custom field types. As a consequence this will give you the chance to binding the grid to your own content types. The only requirement to keep when developing the custom field type is to specify a display mode template and not just a default one.

In the example below, SongName is a custom field type which is connected to a Song list. The field rendering control features a RadComboBox in edit mode that retrieves the names of all songs in the Song list.
 

 Error

 
Web Part Error: One of the properties of the Web Part has an incorrect format. Microsoft SharePoint Foundation cannot deserialize the Web Part. Check the format of the properties and try again.

Error Details:
[WebPartPageUserException: One of the properties of the Web Part has an incorrect format. Microsoft SharePoint Foundation cannot deserialize the Web Part. Check the format of the properties and try again.]
  at Microsoft.SharePoint.WebPartPages.WebPart.ParseXml(XmlReader reader, Type type, String[] links, SPWeb spWeb)
  at Microsoft.SharePoint.WebPartPages.WebPart.ParseCompressed(SPWebPartManager manager, XmlNamespaceManager xmlnsManager, Byte[] personal, Byte[] global, String[] links, Type type, SPWeb spWeb)
  at Microsoft.SharePoint.WebPartPages.BinaryWebPartDeserializer.LoadInitialWebPart()
  at Microsoft.SharePoint.WebPartPages.BinaryWebPartDeserializer.Deserialize()
  at Microsoft.SharePoint.WebPartPages.SPWebPartManager.CreateWebPartsFromRowSetData(Boolean onlyInitializeClosedWebParts)

[Exception: Cannot create XmlSerializers for this Web Part.]
  at Microsoft.SharePoint.WebPartPages.TypeCacheEntry.get_FailedXmlSerializer()
  at Microsoft.SharePoint.WebPartPages.TypeCacheEntry.get_XmlSerializer()
  at Microsoft.SharePoint.WebPartPages.WebPart.ParseXml(XmlReader reader, Type type, String[] links, SPWeb spWeb)